In their long, black cloaks, the eight men and one woman look like crows on a telegraph wire. In reality they are the most powerful judges in America and, on 18 April 2007, they made a decision which opponents fear brings the end of legal abortion in the United States one step closer.

The legal wrangling in the case known as Gonzales v Carhart had gone on for four years. Behind thick walnut doors there was division and anguish. But the outcome was final. By a vote of five to four the American Supreme Court decided to uphold a federal law that allows states to criminalise a certain form of second-trimester termination. It was the first time judges have agreed that a specific abortion procedure could be banned and the first time that they have approved an abortion restriction that does not contain an exception for the health of the woman (although there is an exception to save the woman's life).

Justice Anthony M Kennedy announced the decision before a hushed chamber. While it did not overturn the landmark Roe v Wade legislation which effectively made abortion legal in America in 1973, the ruling marked an unmistakable shift in favour of the rights of the unborn foetus over the rights of the woman. For America's vocal and powerful pro-life movement it was a moment for celebration. For the beleaguered pro-choice contingent this was yet another example of how the rights of women to access safe abortion in America are being whittled away.

One member of the Supreme Court was particularly incensed. The only woman on the bench, 74-year-old Ruth Bader Ginsburg was so angry she was driven to speak out. A deceptively frail-looking woman, she was simmering with rage and steely reason.

'The court deprives women of the right to make an autonomous choice, even at the expense of their safety,' she retaliated. 'The protection of reproductive rights is about a woman's autonomy to decide for herself her life's course, and thus to enjoy equal citizenship stature... This way of protecting women recalls ancient notions about women's place in society .... ideas that have long been discredited.'

Trenchant words from a lawyer who has spent her life campaigning for equal rights for women under the law and who once hid one of her own pregnancies under loose legal clothes for fear of damaging her career. Words, however, that could do nothing to overturn the judgment.

While the arguments may differ across America, one fact is clear: women's access to abortion in the United States is diminishing. Last month a storage company in New York - one of the few pro-choice states left in the country - even made a bleakly comic joke out of it. A new billboard campaign on the corner of 44th Street and Twelfth Avenue shows a picture of a coat hanger next to the words 'Your closet space is shrinking as fast as her right to choose'.

For the last 10 years the pro-life lobby has been winning over the cultural and political establishment. She may still be essentially pro-choice but writing in the New Republic the feminist Naomi Wolf surprised many by her use of language when she suggested 'the abortion-rights movement [must be] willing publicly to mourn the evil - necessary evil though it might be - that is abortion'. Campaigners have lamented that this year's blockbuster comedy Knocked Up about a woman who gets pregnant on a one-night stand barely mentions abortion as an option and in the one scene that it is referred to it is euphemistically called the 'A' word. Even Democrats are nervous about speaking out in favour. Last year Hillary Clinton called abortion a 'sad, even tragic choice'.

Gloria Feldt has spent most of her career battling for the pro-choice side, a position which has meant pickets outside her house and death threats from the opposition. The former chair of Planned Parenthood, she describes 'the Gonzales v Carhart decision like a bungee snapping me back to 1950s west Texas where, as a girl, I absorbed the culture's non-aspirations for women'.

She tells me: 'Gonzales v Carhart marks a seismic shift and one that has not been well recognised. It is basically saying that a woman's health is no longer relevant. For the first time it gives more rights to the foetus than to the woman.'

Most states now allow abortion only up to 13 weeks. Some have imposed mandatory 'cooling-off' periods before the termination can take place, some have laws that require women under 18 to have parental consent. In some areas women are required to undergo an information session during which they will be warned about the possible psychological damage they could suffer, or told the foetus might suffer pain. Should a woman decide to go ahead she must pay around $400 for the procedure.

'These incremental changes to the abortion law have played games with people's minds,' says Feldt. 'You can see people thinking "One more little objection. What can it hurt? Women can still get abortions." The problem is that each restriction on access doesn't seem to impact on the lives of real human beings - until it's you.'

A few months ago Feldt received a call from a Catholic priest. He was anxious to talk to her about better sex education. Two parents in his parish had recently come to visit him. Unbeknown to them, their 14-year-old daughter had got pregnant and had been told by the legal authorities that she must tell her parents if she wanted to go ahead with an abortion. Instead she had found someone who could do the procedure illegally. She died.

'There could be more just like her,' says Feldt. 'But we don't hear from them. It's the most vulnerable who are being affected by this kind of legislation, and it's the most vulnerable who never get their stories heard.'

If abortion has been a vexed and debated topic in America over the last decade, it's about to become one in this country too. But what's worrying British pro-choice groups is the notion that the tactics that have been used to great effect by anti-abortion groups across the Atlantic are already proving successful here as well. So much so, they have decided to go on the offensive for the first time since David Steel's landmark abortion legislation was passed in 1967, exactly 40 years ago.

It means that the next few months could be a tipping point for our abortion debate. As well as the attendant brouhaha around the 40-year anniversary, in November the Human Tissue and Embryo Bill goes through Parliament - and lobbyists on both sides of the argument are planning to hijack it on its way through. An investigation by the Commons science and technology committee into whether medical advances in the care of very premature babies means the 24-week abortion limit should be reduced is also due to report before Christmas.

Both sides claim that they represent the majority of public opinion. Both sides quote statistics and research papers and reports from scientists and doctors. In short, both sides of the abortion argument are gearing up for a fight. The prize is the hearts and minds of young British women who may take the right to abortion for granted. (Complacency, one pro-choice campaigner told me, is his greatest fear.)

Admitting publicly to having had an abortion is still relatively taboo. In the media we're much more likely to hear from the 40-something women who can't get pregnant and regret this fact than the 20-something ones who can and don't want to go ahead. However, it is estimated that one in three women in Britain will at some point in their lives decide to have a termination.

The new face in the pro-life corner is a 36-year-old former television researcher from Manchester called Julia Millington. She is the spokesperson for Alive & Kicking, an alliance of 10 groups who want to see the number of abortions in Britain halved. The group first came to prominence four years ago when they highlighted the case of a woman who had an abortion at 24 weeks because scans showed the unborn baby had a cleft palate.

Where once the pro-life lobby might have been epitomised by intense-looking men in corduroy jackets waving graphic placards, Julia Millington has a friendly 'everywoman' air about her with her long blonde hair and fashionable ballet pumps. Funded by donations, Alive & Kicking is based in a chic office in a Knightsbridge mansion block.

The group's website asks the question: 'Nearly six million abortions since 1967. Happy with that? Neither are we.'

'The overall aim,' she says, 'is to make abortion rare. It is trying to eliminate the need for abortion so that no woman finds herself in a situation where she feels she has no choice but to have one. That cannot be the best that we can do for women. When we talk about a woman's right to choose it's often in the name of women's liberation but in fact it's about as far from being liberated as it is possible to get because you end up having a procedure which at the very least is exceedingly unpleasant and for some has repercussions that last for many years. For many it is not a choice, it is the only option.'

She repeatedly uses the word 'choice', a word that has traditionally been part of the pro-abortion vocabulary. It's a sign of an intriguing shift in the pro-life lobby. Their language used to be aggressive and emotional, the emphasis upon the defenceless unborn foetus. They used to talk about God, and morals, and ethics. Now they're more likely to concentrate on what they see as the rights of the woman. Ignore the bits about abortion and Millington can sometimes sound uncannily like a feminist (in America there's even a voluble group called Feminists for Life who are anti-abortion but argue in favour of better social and educational support for women who find themselves with an unplanned pregnancy).

'Women have abortions for all the same reasons they did in 1967,' says Millington. 'We have not done anything to improve the status of women. We have just made this procedure more hygienic and more socially acceptable.'

But try to pin her down about what she suggests is done to improve the lives of women to such an extent that many more of them feel emotionally, economically and mentally strong enough to go through with an unplanned pregnancy, whatever their age or circumstances, is trickier. 'I don't presume to have the answer,' she says. 'All I know is that whatever we are doing isn't working.'

She means the fact that Britain's abortion statistics are some of the highest in Europe. Last year 193,700 terminations were carried out, an increase of almost four per cent on the previous year. In a bid to reverse these statistics Alive & Kicking proposes some of the same policies that have been approved in America: a reduction of the 24-week time limit, mandatory counselling, an enforced cooling-off period. Their prime concern is so-called conveyor-belt abortions. In exactly the same way as has happened in America, the strategy is incremental changes which, taken in isolation, might sound reasonable - even to those who support a woman's right to choose.

But behind these proposals the group remains against abortion of any kind. 'We would never draw a line in the sand before which life has less value than after, because we are equally opposed to abortion in early pregnancy too,' she says.

Ann Winterton, Conservative MP for Congleton, is one of three MPs - all of them female - who have introduced 10-minute rule bills over the last 12 months seeking in some way to re-address the 1967 legislation. She is against so-called social abortions ('I'm not Roman Catholic; it's an instinctive thing') and wants women to be given mandatory counselling before termination procedures. 'One woman told me, "I had a pregnancy test on Monday, a scan on Tuesday, an abortion on Wednesday. No one asked me if I was sure." I wanted to address what I see as the lack of information, back-up and support given to women when they go for an abortion.'

She was inspired partly by a close friend who had an abortion when she was younger and has suffered periodic depression ever since. 'No one chooses to have an abortion without some soul-searching. It is a horrific decision to make. It runs deep and the effect goes on for a very long time.'

Winterton never expected the bill to succeed - with a large pro-choice majority in the House of Commons, including prime minister Gordon Brown, the bill was defeated by 75 votes - but in many ways that did not matter. It did its job by giving another hearing to the pro-life lobby's view that abortion causes long-term psychological damage and reinforcing the idea that they have the woman's best interests at heart.

She sees no contradiction in the idea that, on the one hand, pro-life groups want shorter time limits, and on the other they want women to wait longer before they make a decision. 'The last 40 years have been a horrific story,' says Winterton, sounding increasingly irate. 'I don't think it has done women any good at all. We are on a slope whereby if something becomes available, it becomes more available. We virtually have abortion on demand in this country. Finally the penny has dropped. These are not blobs of jelly that can be flushed down the lavatory.'

In smart, glass-fronted new offices near the British Telecom tower in central London, Tony Kerridge is planning a counterattack. He's the spokesperson for Marie Stopes International, the organisation which carries out about a third of all the abortions in Britain, both privately and under licence to the NHS. He's in the middle of arranging a conference titled Safe Abortion: Whose Right? Whose Choice? Who Cares?, a conference he has described as 'a call to action'.

Since beginning work in this field 11 years ago he's become increasingly concerned about the diminishing proportion of media coverage the pro-choice lobby has received. 'It's time to go on the offensive. Since the 1967 act we've played a defensive role and too often we've felt on the back foot. It's time to take a more positive stance, to create modern legislation for the 21st century.'

Anne Quesney, a former teacher and the chair of the campaign group Abortion Rights, was born in Belgium where abortion was only made legal in 1990. When she was growing up young women would travel to Holland to have a termination. 'There is a sense at the moment that women are harmed by abortion but in my experience women who cannot access safe legal abortion are in much more trouble than the women who can. The situation in the States has very much shaped the anti-abortion rhetoric in this country. They are using the same hot buttons - late abortion, parental concern, psychological harm.

'In the Fifties the anti-choice lobby said masturbation made you blind. We've worked out that's not true. Then that abortion made you infertile, which has been discounted. Then that abortion gives you breast cancer. Also disproved. Now they say abortion causes psychological trauma. We all know women who have had an abortion. Is it really having such a devastating effect?'

One of the speakers at the conference is the columnist Suzanne Moore, one of the few female journalists who has written about her own abortion when she was younger. 'Believe most of the media,' she says, 'and you'd imagine we are all binge-drinking, having sex, getting pregnant, leaving it late and going out in our lunchtime to have our babies killed.' For her, denying women the right to an abortion is a type of fundamentalism. 'Objecting to abortion means controlling women's bodies. This is fundamentalist, it just isn't Muslim fundamentalism.'

Marie Stopes International, Abortion Rights and 11 other 'Voice for Choice' organisations have put together their own set of proposals which directly counterbalance those on the pro-life side. As well as extending abortion to Northern Ireland where it is still illegal, they want to see quicker and easier access by ending the legal stipulation that there must be two doctors' signatures before an abortion can go ahead (abortion is the only medical procedure that requires the consent of two medical experts) and the introduction of legislation which would mean a nurse could conduct the procedure in the first trimester.

According to Kerridge, Britain - once the pioneer in abortion rights - has fallen behind much of the rest of Europe. 'At the time it was an amazing piece of legislation, absolutely fantastic. But don't forget,' he says, 'the 1967 law was passed when most doctors were men. It was empowering men to have a hold over women's decision-making and autonomy over their bodies.' Eighty-nine per cent of abortions already happen before 13 weeks; a termination in the first three months is no more challenging than fitting a coil, he argues. 'What women want is something that is quick, over and done with, back on with my life, no fuss.'

If anything has harmed their cause over the last five years it would be the ultrasound images from Stuart Campbell, former professor of obstetrics and gynaecology at King's College London. The arresting pictures of a 12-week-old foetus, repeatedly described as 'walking' or 'sucking its thumb', proved irresistible to newspaper editors and provoked a new debate around viability despite the fact that neonatal experts said the scans, while compelling, revealed nothing new medically.

'Every woman who has a termination knows what she is doing,' says Kerridge. 'She knows that if she doesn't interrupt the pregnancy there will be a baby and it is patronising to think that is going to change because we have more sophisticated ways of looking at the images. An unwanted pregnancy is an unwanted pregnancy. End of story.'

What would be disastrous for the pro-choice side is a reduction in the time limit. Campaigners argue that these later abortions are both rare and often the most vulnerable cases. These are women in extreme and desperate situations, says Quesney. The victim of domestic violence, the teenager who refuses to believe she is pregnant, poorer women who can not afford the £400 to go privately. Campaigners argue that, unlike many other countries in Europe, we don't have abortion on demand, far from it. She recently heard of a woman who had an abortion at 21 weeks who had originally presented at her GP's surgery at five weeks pregnant. Another came up against so many delays from her pro-life doctor - around five per cent are said to actively deter women from having abortions - that she ended up having to carry on with the pregnancy.

'The very few women who make the decision to have a late abortion do so for very good reasons,' says Quesney. 'It is not an easy decision.' It's a view backed up by the British Medical Association who have voted to retain the 24-week time limit.

'The anti-abortion lot cherry-pick their causes,' says Kerridge. 'They go for what they see are the most vulnerable elements. Things like the 24-week time limit despite the fact that that is only one per cent of all abortions.... The fear is that the 24-week aspect will suffer because people are taken in by the one child in how ever many thousands who survives without profound physical and mental difficulties at 23 weeks.'

Suzanne Moore agrees. She believes an attack on late abortion is an attack on all abortion. 'Know your enemy,' she says. 'The pro-lifers have spotted the weak link - the moderates who think abortion up to 12 weeks is OK. Everybody has to wake up and understand that in this country we don't want the kind of culture wars they have had in the US. Don't imagine it won't happen. The anti groups will try to do it, by whatever means necessary.'