Ministers have seen off a backbench rebellion by promising concessions on a bill which critics claim would legalise euthanasia "through the back door".
Diane Pretty's case sparked more debate about euthanasia
The Mental Capacity Bill formalises "living wills" allowing people in Wales and England to say they want treatment stopped if they fall seriously ill.
During MPs' debate, it emerged the lord chancellor had pledged to put measures in the bill preventing killing.
MPs were angry they had not been told of the move as they prepared to vote.
'Killing by omission'
The bill allows people to give somebody the power of attorney to make decisions on their behalf if they become too ill to decide for themselves.
Ministers insist the plans would not change laws on euthanasia and would improve safeguards.
Christian groups and other critics fear it could allow "killing by omission" through withdrawing treatment, including food and fluids.
That point was pressed home by backbenchers on all sides of the Commons.
Labour MP Brian Iddon said: "If people don't believe that the people who are pushing living wills are not going to push assisted suicide or voluntary euthanasia then they are living in cloud cuckoo land."
Ex-Conservative leader Iain Duncan Smith warned that people making living wills could have changed their mind by the time they were incapacitated.
His attempt to ensure the bill did not allow decisions aimed at causing death was defeated by 297 to 203.
But 34 Labour MPs defied government orders to oppose the amendment and Mr Duncan Smith said about another 100 had abstained.
The bill was later given an unopposed third reading by 354 votes to 118, a government majority of 236.
The government staved off defeat only after news emerged of a pledge to strengthen safeguards against killing, contained in a letter from the lord chancellor to the Catholic Archbishop of Cardiff, the Most Reverend Peter Smith.
Lord Falconer said he would try to make it explicit in the bill that it did not authorise any decision aimed at killing, but was aimed at relieving suffering or ending treatment where the patient was in an irreversible coma.
Archbishop Smith said he wanted to see the details of any changes, telling BBC News: "In principle, I think they have conceded the point."
MPs complained of a "farce" as Constitutional Affairs Minister David Lammy faced a barrage of calls to show them the Lord Falconer's letter.
Amid rowdy scenes, Mr Lammy was eventually handed a copy of the letter to read out.
He insisted that the assurances given by the lord chancellor would "fully meet the concerns" of those who
He argued the government did not want to change the law but to strengthen it adding that both doctors and patients said they wanted "more clarity".
But some MPs privately said Mr Lammy had been out of his depth and deputy speaker Michael Lord said the government had not organised the debate as it should have done.
BBC political editor Andrew Marr said after the chaotic scenes in the Commons, Mr Lammy was "waste deep in quicksand and sinking fast".
"They've got the bill through, it now goes to the Lords but it goes to the Lords after a hard and inglorious afternoon."