Ministers have promised to change a bill which critics claim would legalise euthanasia "through the back door" so that it cannot authorise killing.
Diane Pretty's case sparked more debate about euthanasia
The bill would give legal force to "living wills", where people say they want medical treatment withheld if they become severely incapacitated.
Some Christian groups say it could mean doctors withholding food and fluids.
The government has now pledged to make explicit in the bill that it does not allow decisions aimed at killing.
'Killing by omission'
Ministers insist the Mental Capacity Bill - for England and Wales - would not change laws on assisted suicide and that it favours the preservation of life.
Critics fear it could allow "killing by omission" through withdrawing treatment.
Lord Falconer has now written to the Archbishop of Cardiff, Peter Smith, saying the bill is not meant to authorise any decision where the motive is to kill as opposed to relieve or prevent suffering or ending treatment where the patient is in an irreversible coma.
Ministers would now try to make that intention explicit in the bill.
Archbishop Smith said he wanted to see the details of any changes but told BBC News: "In principle, I think they have conceded the point. There was a lot of misunderstanding."
MPs complained of "farce" because they had not seen the letter as they prepared to vote on a series of backbenchers' amendments.
Eventually Constitutional Affairs Minister David Lammy read the letter and confirmed the lord chancellor's promise would cover acts of omission of treatment, including food and fluids.
He said the government did not want to change the law but to strengthen it.
"Doctors are saying they want more clarity," said Mr Lammy. "Patients are saying they want more clarity."
The bill would establish a legal presumption that everybody is able to make decisions about their own treatment unless they are proved to be mentally incapable of doing so.
It would also allow people to give somebody the power of attorney to make decisions on their behalf, which could be challenged by doctors.
The bill is currently in its report stage and will go to a third reading and be debated in the Lords before it can become law.
Christian groups have said the bill would allow euthanasia by the "back door".
Peter Saunders of the Christian Medical Fellowship said: "CMF is concerned that patients will make unwise and hasty advance decisions to refuse food and fluids without being properly informed about the diagnosis
and the expected course their illness will take."
In the Commons, ex-Labour minister Frank Field said those wanting changes to the bill did not want to keep people alive at all costs.
"We don't want to bop people off when they have actually got quite a lot of life in front of them," he said.
Ex-Conservative leader Iain Duncan Smith urged MPs to "find for life", saying people making living wills could have changed their mind by the time they were incapacitated.
The Making Decisions Alliance, which includes the Alzheimer's Society and Age Concern, said the bill did not change the current law but offered better safeguards.
Debate on legalising euthanasia has intensified in the UK because of cases like that of motor neurone patient Diane Pretty.
She died two years ago after losing a legal battle to allow her husband to help her commit suicide.