Safeguards to protect sick patients who set up 'living wills' dictating their future medical treatment are being drawn up by ministers in an attempt to reassure campaigners that the plan will not lead to euthanasia by the back door.

Pro-life lobbyists have campaigned fiercely against the government's Mental Capacity Bill, which would give legal force to directives allowing patients to dictate how they want to be treated when they can no longer communicate for themselves.

Campaigners argue that unscrupulous relatives who might gain from the person's death could exploit such directives.

Patients' groups however warned this weekend that new rights for Alzheimer's sufferers and the disabled contained in the bill are now being put at risk by the anti-euthanasia lobby misguidedly 'hijacking' the argument.

Baroness Ashton, the constitutional affairs minister, told The Observer she was 'keen' on a compromise put forward by George Howarth, a Labour MP, 'essentially saying that when you look at end-of-life treatment, you begin from the starting point that people want to be alive'.

While the bill makes clear that doctors who are unclear about a patient's advance directive - or suspect they have changed their mind since making it - should treat them, the Howarth amendment would clarify that presumption of life.

'The tests and the hurdles that you have to go through [before withholding treatment] are much, much stronger than has been suggested,' said Ashton, who met concerned MPs last week to try to defuse a revolt when the bill comes back to the Commons on Tuesday.

'At the end of the day, we think it's right that people can appoint someone they love to be their representative to speak for them when they no longer can themselves,' Ashton said.

'That is a better option for some people than a doctor who may be wonderful but he doesn't know them.'

Patients who drew up living wills would grant 'power of attorney' to a child, spouse or trusted relative to enforce their wishes once they became too ill to communicate: at the moment, such powers can only be assumed by relatives over financial affairs.

Ashton said the bill would also protect doctors who gave emergency treatment to an unconscious person, only to find themselves sued when the patient woke up and challenged the treatment given.

'If you are taken in from a road accident and you are unconscious, and the doctor fixes you up, he actually could be committing assault and battery. There is nothing in law that protects him.

'There are cases beginning to appear in the United States, for example patients who don't want blood transfusions because of religious beliefs who get them.'

Almost eight in 10 Britons currently signing advance directives are Jehovah's Witnesses whose faith does not allow transfusions.

Ashton was backed up by an alliance of 39 medical charities supporting the bill. They argue that the bill explictly states that nothing in it alters the law on euthanasia, currently illegal in Britain, and that new rights for the sick and disabled to assert themselves would also be lost if the bill was defeated.

'At the moment there is no confidence that someone you know well and want to entrust your decisions to will even be asked for their views,' said Julia Cream, the head of public affairs at the Alzheimer's Society. 'We believe this bill has been completely hijacked around the debate about euthanasia.'

While campaigners want living wills to stop short of demanding that food and drink be withheld, Cream said that could be inhumane: 'If someone cannot swallow because their dementia is so far advanced, and cannot be given sips of water, and what is being proposed is a tube through the stomach, research shows that people find that distressing.'