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.

“We were elected in to change what is broken in our country,” he added.

Sir Keir said 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.”