Changes to a bill which critics claim would allow "back door" euthanasia will not allow deliberate killing, Tony Blair has told MPs.
Mr Blair defended the handling of the Mental Capacity Bill
Ministers have been accused of panic for offering last minute changes to the Mental Capacity Bill amid chaotic scenes in the Commons on Tuesday.
Mr Blair defended the handling of the issue at prime minister's questions on Wednesday.
He said critics' concerns would be tackled when peers debate the bill.
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, or make "living wills" about withholding treatment.
Ministers insist the plans would not change laws on euthanasia but would improve safeguards.
Critics fear it could allow "killing by omission" by allowing treatment, including food and fluids, to be withdrawn.
Mr Blair denied the issue had been "weakly" handed to the Lords to clear up, saying it was sensible to address people's concerns.
But he said changes to the bill must not overturn the law set when a court ruled that doctors could withdraw artificial feeding and hydration from Hillsborough coma victim Tony Bland.
"At the House of Lords stage we will table amendments that will make it absolutely clear that while the Bland judgment will remain in being, we will not in any shape or form countenance the deliberate killing of people," he said.
The bill was needed to prevent confusion about the law among doctors who might otherwise lay themselves open to prosecutions which no sensible person would want.
Lord Falconer has promised a new clause saying nothing in the bill authorises any act where the motive of the person taking the decision is to end life.
"The motive has got to be to end suffering," he told BBC News.
He denied he was embarrassed over the handling of the bill and praised the "excellent job" of his junior minister, David Lammy, who some MPs have accused of creating a shambles.
Lord Falconer admitted problems were caused by him offering the promise so late during the debate.
On Tuesday, the government saw off a backbench attempt to force changes to the bill by 297 votes to 203, despite rebellion by 34 Labour MPs.
The revolt was also reduced by news Lord Falconer had promised the Catholic archbishop of Cardiff to strengthen safeguards in the bill.
But that only came after MPs bombarded Mr Lammy with a barrage of requests for him to read the letter as they complained they had been left in the dark.
Eventually, he was hurriedly handed the letter to read out five minutes before the crunch votes, prompting claims of a shambles.
Speaker Michael Martin said he would investigate claims the letter was distributed to Labour MPs against Commons rules.
Ex-Tory leader Iain Duncan Smith said the debate was a "charade" and complained the promises of changes to the bill were vague.