Sam Francis
Political reporter
PA Media
Sir Keir Starmer has vowed to "press ahead" with the government's planned welfare changes, despite a growing rebellion from Labour backbench MPs.
More than 120 Labour MPs have signed up to an effort to block plans to cut disability and sickness-related benefits payments to save £5bn a year by 2030.
The threatened rebellion is enough to wipe out the government's working majority in Parliament.
But speaking ahead of a meeting of Nato leaders, the prime minister said the current welfare system was "unsustainable" and could not be left unreformed.
Asked by journalists if he would consider pausing the reforms given the size of the rebellion, Sir Keir said: "I intend to press ahead".
He added that the current welfare system "traps people" on benefits, and was set to fuel "unsustainable" rises in the cost to taxpayers.
He added that the projected increase in the number claiming Personal Independence Payments (Pips) each year was "the equivalent of the population of a city the size of Leicester".
"So those that care about a future welfare system have to answer the question: 'how do you reform what you've got, to make sure it's sustainable for the future?"
Asked if he would be happy to rely on Conservative votes to push the reforms through Parliament, he said: "I have no idea what the Conservatives will do. I don't think they've got the first idea what they're doing."
'Groundswell of opinion'
The Universal Credit and Personal Independence Payment Bill - set to be voted on by MPs next Tuesday - would make it harder for disabled people with less severe conditions to claim personal independence payment (Pip).
The rebel Labour MPs have signed a so-called "reasoned" amendment that, if selected by Commons Speaker Sir Lindsay Hoyle and approved in a vote, would stop the bill progressing through Parliament.
The rebels cite the the number of people the plans are expected to push into relative poverty, a lack of consultation, and an inadequate impact assessment on the consequences on the jobs market as reasons why they oppose the plans.
While the success of the amendment is not assured, over half of all Labour backbenchers have signed it, indicating the extent of the potential rebellion facing ministers.
Labour MP Dame Meg Hillier, who tabled the amendment, has called on the government to "listen to this groundswell of opinion" and change course.
Dame Meg, who also chairs the Treasury Select Committee, told BBC Radio 4's World at One the government's communication around the bill has "not been good" and had led to confusion.
While agreeing that the welfare system needs reform, she warned there were flaws in how the plans were being rolled out, and said Labour experts were shut out of the process.
One of the main co-ordinators behind the amendment, who did not wish to be named, has told the BBC the government's U-turn on cutting winter fuel payments had emboldened many of those who have signed the amendment.
They said MPs "all voted for winter fuel [cuts] and have taken so much grief in our constituencies, so colleagues think why should I take that on again?".
It is understood that plans for the amendment began when Work and Pensions Secretary Liz Kendall offered a partial olive branch to rebels by expanding the transition period for anyone losing Pip from four to 13 weeks.