Last year, my mother-in-law fell off her Stannah and broke a hip. If that sounds like the cue for a Les Dawson joke ("I was hoping it would be her neck"), it isn't: I'm fond of my mother-in-law and the result of her little accident, not funny in the least, was that she nearly died. After a belated but successful operation, she developed c diff (clostridium difficile, the nation's favourite hospital killer bug after MRSA), and three courses of antibiotics failed to clear the infection.

Suddenly a robust, cheerful woman of 79, whose only mobility problem had been climbing stairs, began to talk of never leaving hospital again but of being "laid out on a marble slab" - and when the hospital asked for our permission not to resuscitate her should she lose consciousness, we realised this was no paranoid fantasy.

Thanks to the efforts of her five children, who travelled long distances to see her, brought food, pleaded with staff not to write her off and eventually - because the pleas were falling on deaf ears - moved her to another (I'm afraid, private) hospital, my mother-in-law is still around, less active than she used to be, but alive to see in another new year. She was lucky. We were lucky. But as I'm beginning to discover, many people with elderly relatives are not.

Pauline Pringle wasn't lucky. Her mother Sarah Ingham died around the same time and in similar circumstances, on January 6 last year: a badly dislocated hip was missed and after the operation that eventually followed, Sarah spent 12 weeks in Tameside hospital, Manchester, failing to shake off a post-operative infection and - denied a proper diet - losing three and a half stone in weight. She was then sent home, where the local GP knew nothing of her discharge and didn't recognise her as the same woman he'd seen three months earlier. She died within a fortnight. At the inquest the coroner, John Pollard, said that he would be writing to the hospital to demand an explanation for Sarah's malnourishment: "It is totally unsatisfactory in a major city in a western democracy that families have to bring food into a hospital because their loved ones are not being fed properly by staff."

Pauline is a witty, ebullient woman and - on the principle that if she couldn't joke about it, she'd weep - she laughs about the four mattresses sent to her home in anticipation of her mother's discharge (one smelled of shit, another was an inflatable with a puncture). But it's the malnutrition issue that preoccupies her. Because her mother was too weak to chew solids, she was put on a "soft diet" of pureed food and "build-up" drinks. But staff disregarded the nutrition regime and brought her food she couldn't eat, including, on one occasion, fish and chips.

"I'd point out to the nurses that she'd been given the wrong thing," Pauline says, "and they'd say, 'Oh, well, we'll fetch her an ice cream then.' But how much nutrition is there in an ice cream? How can ice cream give you the strength to fight off an infection? In the end I'd mash up things at home and bring them in. I used to take little gifts for the staff as well - as a way of saying, 'This is from Sarah, over there in that bed, will you please feed her.' I'd have done double-flips backwards through burning hoops if that's what it took.

"Of course, my mum would always say, 'Don't make a fuss,' and a lot of patients and relatives on the ward were the same - afraid they'd have an even worse time of it if they caused trouble. But after she died, when the inquest was reported in the local papers, people began coming up to me in the bank or the supermarket saying, 'The same thing happened to me.'"

Pauline works as a carer herself, and understands the problems of understaffing and under-resourcing. But she's incensed by the hospital's refusal to acknowledge responsibility. "Nothing can bring my mother back or compensate. But I want them to admit they were negligent. I want truth and I want closure." It's not a witch-hunt against Tameside, she says, but a campaign for improvement throughout the NHS. "What happened to my mother could have happened - does happen - anywhere in Britain. It's not just a few stray buds, it's a forest. And it's not just the poor who are affected, it could be anyone."

The experience of Norman Irons, former Lord Provost of Edinburgh, proves her point. His mother, Anne, was admitted to Edinburgh Royal Infirmary's orthopaedic ward after breaking a pelvis. Though 89, she was otherwise in good health; all she needed was rest and nursing. Instead, she was treated as a "bed blocker", her basic hygiene neglected (in 28 days on the ward she was never given a bath) and her food and drink intake poorly monitored. She eventually died of bronchopneumonia, but her son believes that loss of morale was the underlying cause. "Her head went down along with her weight ... No effort was made to establish what food she liked or could eat, to spend time and encourage her, or to get any special diet. When she died her upper arms were thinner than a broompole."

Though an apology to Norman Irons was slow in coming - and, but for the daughter of a woman on the same orthopaedic ward making the same complaint might never have come at all - the Lothian health authority now admits that it failed his mother and has embarked on an action plan to improve its care of the elderly: better quality food, nutritional training for nurses, and the creation of "modern matrons" are among its priorities.

If such reforms were implemented nationwide, they would bring immediate improvements. But the deeper problem, less easily eradicable, is society's perception of the old. The kind of logic nurses resort to when they're overstretched - there's a lady in bed three whose drink is out of reach, and who needs her spectacles in order to see what she's eating, but I've a whole ward to attend to and, what the hell, she's going to die anyway - will seem familiar to most of us. It's not only in hospitals that old people are ignored, written off and abused.

One much quoted figure suggests that up to half a million elderly people in the UK are being abused at any one time. Unlike child abuse, elder abuse is rarely reported beyond local newspapers, and those who inflict it are less likely to be held to account.

When a House of Commons health committee produced its report, Elder Abuse, in 2004, it suggested that "abuse in domiciliary settings is the commonest type". Overall, though, whereas the old are more likely to be robbed of money or possessions by their nearest and dearest, they're more likely to starve to death in a hospital or care home. Dr Adrian Treloar, a specialist in geriatric psychiatry, caused a furore in 1999 when he applied the phrase "involuntary euthanasia" to the way in which elderly patients in NHS hospitals were being deprived of food and water and "left at the bottom of the pile".

The then health minister John Hutton denounced the allegation as "ludicrous and scaremongering". But it's not a new allegation, nor do most of us find it hard to understand how a person of 90 might be given less priority, to put it at its lowest, than a child of five.

Shakespeare spoke of "unregarded age in corners thrown" (As You Like It), and in 1967 Barbara Robb published a book called Sans Everything: A Case to Answer which explored the marginalisation and mistreatment of the elderly. But whereas in most material respects British society has prospered since the 1960s, our care of the elderly seems to be getting worse. Privatisation within the NHS has only added to the problem: now that patients are seen as consumers, the old have become an underclass - degenerates or unregenerates whose continued existence doesn't merit much investment.

When old people are suffering from Alzheimer's disease or multi-infarct dementia, they're even more vulnerable. In his new book, Untold Stories, Alan Bennett writes movingly about his mother, who suffered from depression and spent 25 years in and out of hospitals, mental institutions and homes, the last of them in Weston-super-Mare, where she shared a room overlooking the sea with four other women, all in the last stages of dementia. He describes one of his visits there: "None of these lost women can feed herself and to feed them properly, to spoon in sufficient mince and mashed carrot topped off with rhubarb and custard to keep them going, demands the personal attention of a helper, in effect one helper per person. Lacking such one-to-one care, these helpless creatures slowly and quite respectably starve to death. This is not something anybody acknowledges, not the matron or the relatives (if, as is rare, they visit), and not the doctor who makes out the death certificates. But it is so."

Bennett writes acceptingly of the situation: "Whereas a newspaper might make a horror story of it," he says, "I can't." In his situation, I'd feel the same: sometimes there's a kindness in allowing old people to slip away. But as Bennett says, one should be clear that this is what goes on. And when the policy is misapplied, consigning those who might recover to a premature death, or when it's accompanied by other less kindly forms of neglect (filth, rough handling, use of "the liquid cosh"), then it becomes less easy to forgive.

Malnourishment isn't just a deprivation in itself, but lowers the immune system, slows recovery, and increases the risk of depression, anaemia, secondary infection and bedsores. Scottish executive figures for last year showed that almost 1,700 patients diagnosed with malnutrition had recently been discharged from hospital, and there's no reason to think that hospitals in England and Wales do any better. The growth of pressure groups is one measure of the problem. The organisation Patient Protect, for example, was set up in the 1990s by a dentist in Kent, Roger Green, in memory of his mother Margaret, who on the morning of her admission to hospital with an abdominal problem had travelled by bus to do voluntary work for Oxfam.

Though she was not suffering from a terminal illness, DNR (Do Not Resuscitate) was written in her notes and a "Nil by mouth" sign hung over her bed, and with "a negative fluid balance of over 4.5 litres" she duly died from gross dehydration. His website conjures a nightmare world in which patients die having been prescribed with (coma-inducing) diamorphine or (dehydration-inducing) diuretics. It also gives solid practical advice on how not to become a victim of "rationing", whether of staff, equipment or diet. "Always keep a notebook and pen handy," it advises. "Ask at the nurses' desk which nurses are responsible for hydration, nutrition and pain control ... Relatives who do not know the signs of dehydration may be tricked into believing that the rapid deterioration is due simply to the underlying illness."

Anni Bales, who also lives in Kent, certainly believes that she was duped in this way. It's three years now since her father died, but her voice breaks when she talks about it: "I've experienced things that are almost beyond belief. It never goes away." Her father Charles, a retired policeman, suffered from dementia. It was when he used garden shears to cut through the electric cable to the cooker, because he thought it was a telephone connection, that he was sectioned and admitted to hospital.

The hospital was bad enough, says Anni. Worse still was the care home he moved on to after four months: the staff there had been told Charles was "normal" (on the grounds that he walked round carrying books, although he never read them), and, ill-equipped to cope with his dementia, they resorted to removing the light bulb from his room to stop him wandering. Soon enough they wanted him out.

That's when the real problems began, in the second care home, which supposedly specialised in cases such as his. It wasn't the faeces in the bedside drawer that worried Anni so much, the untrained staff, the lack of records or the fact that new clothes they bought for him went missing. It was his loss of weight. Asked if he wanted to eat, he would say no. But by that time he said no to everything (it was the only word he used, and one whose meaning he no longer understood); sit down and offer him food and he would eat.

At some level, Anni thinks, he knew he was hungry; that's why he hid it for later, like a squirrel. But he had forgotten how to eat for himself. And no one on the staff had the time to help. It was the same with drinks. Anni remembers arriving on a hot summer day and offering him one - he drank and drank like a man finding water in the desert.

By the time he was re-admitted to hospital, he'd lost five stone and was covered in bruises and bedsores. "He looked like something out of Belsen," Anni says. "He'd been left starving to death in a filthy room. I'm a biomedical scientist, I've worked in hospitals most of my life, but I don't know much about dementia and when they told us that people with dementia lose the ability to swallow I believed them. I feel so stupid now, and so guilty. Of course, it is very difficult to look after mentally troubled people, I don't deny that. It's an unglamorous end of the profession, so the better doctors and nurses keep moving on - there's no continuity. All the same, it's not right that my father should have paid the price."

Sudden, catastrophic declines are common in old age; sometimes the body has simply had enough, and its lack of interest in replenishing itself is a means of securing an easeful death. But this wasn't the case with Anni Bales's father. Back in hospital - an old-fashioned Victorian hospital this time - he put on weight and recaptured some of his old humour. When he died, it was from a blood clot, not malnutrition.

It's not that Anni is unable to "let go", but that she knows his treatment in the care home was unacceptable. "My father was a proud man, a soldier who swept bodies off the D-Day beaches, and I often think: what if he'd known, what if he'd taken it in? If it was a dog, the RSPCA would prosecute. But when it's an old person, no one wants to know. The attitude is, this is Britain, things like that don't happen here."

In his poem, To the Sea, Philip Larkin describes holidaying British families: Coming to the water clumsily undressed / Yearly; teaching their children by a sort / Of clowning; helping the old, too, as they ought.

A sense of obligation to our parents and grandparents grows stronger for most of us as we get older, and if we fall short, or those appointed on our behalf do, then it's forever on our conscience. That's why we need to do better by the elderly: for our sake, as well as theirs. And because, one day, it will be us.