Keir Starmer is battling to avoid the biggest Labour rebellion since he became prime minister after the government published details of its plan to slash £5 billion from the benefit bill.

Work and pensions secretary Liz Kendall insisted the Universal Credit and Personal Independence Payment Bill “marks the moment we take the road of compassion, opportunity and dignity”.

But campaigners said the legislation would be “a disaster for disabled people”, while rebel Labour MPs said attempts by the government to soften its impact on benefit claimants did not go far enough.

The government says its aim is to get more people off sickness benefits and into work by making it harder for people to claim personal independence payments (PIP) and cutting the sickness-related element of universal credit (UC).

But the government’s own assessment found that the changes will push 250,000 people – including 50,000 children – into poverty.

More than 100 Labour MPs have signed letters to the government’s chief whip making clear their opposition to the plans.

A Commons vote on the plans will take place within weeks.

In a bid to address the rebels’ concerns, the government has announced that those claimants who are set to lose out will continue to receive their money for 13 weeks.

Kendall said: “Our social security system is at a crossroads. Unless we reform it, more people will be denied opportunities, and it may not be there for those who need it.

“This legislation represents a new social contract and marks the moment we take the road of compassion, opportunity and dignity.

“This will give people peace of mind, while also fixing our broken social security system so it supports those who can work to do so while protecting those who cannot – putting welfare spending on a more sustainable path to unlock growth.”

But one Labour rebel told HuffPost UK the government’s attempts to make the bill more palatable to its opponents were insufficient.

He said: “Saying benefit claimants can keep getting their money for 13 weeks before having it taken away is like giving someone a redundancy payment – it doesn’t change the fact that you’re still losing your job.

“At the very least there are 100 Labour MPs who are opposed to the bill, and I’ve seen nothing from the government that is likely to reduce that number.

“The whips and ministers will now start reaching out and trying to persuade people to at the very least not vote against the bill, but I think we’re still looking at a sizeable rebellion.”

Another rebel MP said opposition to the bill is “hardening”.

“Lots of new MPs are gearing up to actually oppose the bill rather than just abstain, which is very interesting,” the MP said. “The chief whip has now decided that threatening people is unhelpful – horse and bolted spring to mind.”

Siân Berry, the Green MP for Brighton Pavillion, said: “With this bill the government is making a cruel, unnecessary and callous political choice.

“Recklessly pushing this bill through parliament shows that ministers are still not listening to the millions of disabled people who will be catastrophically affected by these reforms about how their daily lives really work.”

Charities representing those affected by the bill have also condemned the government.

Mark Rowland, chief executive at the Mental Health Foundation, said: “This bill is a disaster for disabled people, and we urge MPs to reject this legislation in its current form.

“It will take vital financial support away from hundreds of thousands of disabled people, many of whom have mental health problems. Previous austerity measures worsened people’s living conditions, undermined their mental health and increased the risk of suicide and premature deaths – and that’s likely to be the case here too.

“These plans will not help the UK government reduce the number of disabled people out of work. PIP is also granted to disabled people who are in work to help them meet the cost of their disability, and removing this support will make it harder for them to remain in the workforce.

“The Office for Budget Responsibility has also said there is no robust analysis showing this will encourage anyone to return to the workforce. Given that, our view is that these cuts are counterproductive and cruel.”