A row is brewing over planned new laws which critics claim would allow euthanasia by the back door.
Diane Pretty's case sparked more debate about euthanasia
MPs debate the detail of the Mental Capacity Bill on Tuesday.
The bill would allow people to draft "living wills" to say they want medical treatment withheld if they become severely incapacitated.
Ministers insist the bill would not change laws on assisted suicide and would protect patients unable to make decisions themselves.
Killing by omission?
The Mental Capacity Bill would establish a legal presumption that everybody can make decisions about treatment unless proved otherwise.
It would allow people to give somebody the power of attorney to make decisions on their behalf.
Doctors could apply to a court of protection to challenge the decision.
Baroness Warnock says she would consider euthanasia
Critics fear the plans could allow "killing by omission" through withdrawing treatment, which they claim could include food and fluids.
Ninety-one MPs signed a petition begun by ex-Conservative leader Iain Duncan Smith saying the bill should specifically prevent decisions to bring about death.
That call will be pressed in the Commons, where Conservative and Liberal Democrat MPs will have a free vote. Tory spokesman Tim Boswell will support Mr Duncan-Smith's amendment.
Saying the current law was unclear, Lib Dem spokesman Paul Burstow said:
"Decisions about withdrawing treatment should be a matter for the Courts to decide on a case by case basis."
'I want to live'
Labour MP Claire Curtis-Thomas is among those worried about the plans, saying incapacitated people can change their minds.
She has told how her mother, after suffering a stroke, signed a directive saying she wanted to be helped to die if she became incapacitated again.
But five years later in a similar situation she had changed her mind and "blinked out, 'I want to live'".
Ministers insist the plans do not sanction euthanasia. There would be safeguards for doctors asked to remove a life support machine.
Minister David Lammy said there was a clear presumption in favour of the preservation of life.
Mr Lammy said changes were needed to outdated and obscure laws which affected the 750,000 people with dementia and the five million people in England and Wales who were carers for people without mental capacity.
The Making Decisions Alliance, which includes the Alzheimer's Society, Age
Concern, Mencap and the National Autistic Society among its members, called the euthanasia fears "misplaced".
It claimed: "It will not change the current law on euthanasia and will
actually provide a series of better safeguards when decisions are made for
people who lack capacity."
The British Medical Association also backs the bill, saying it just gives incapacitated people the same rights as others.
Medical ethics expert Baroness Warnock has told The Sunday Times she would consider euthanasia if she became a "nuisance" in old age, sparking anger from charities.
Lady Warnock was part of a Lords committee which opposed euthanasia over a decade ago but says she was moved by the case of Diane Pretty.
Mrs Pretty suffered from motor neurone disease and unsuccessfully fought a legal battle to allow her husband to help her take her life.