Labour MP Vicky Foxcroft has resigned as a whip over the government’s plans to cut disability benefits.

In a letter to the prime minister, Foxcroft said she understood the need to address “the ever-increasing welfare bill” but said cuts to personal independence payments and universal credit should “not be part of the solution”.

She said she had “wrestled with whether I should resign or remain in the government and fight for changes from within. Sadly it now seems that we are not going to get the changes I desperately wanted to see.”

Responding to her letter, a government spokesman said it was fixing a “broken welfare system” that was failing the sick and vulnerable.

“Our principled reforms will ensure those who can work should, that those who want to work are properly supported, and that those with the most severe disabilities and health conditions are protected.”

Earlier this week, the government published its bill, which tightens the criteria people have to meet in order to get personal independence payments (Pips) and cuts the sickness-related element of universal credit.

More than 100 Labour MPs have expressed concern about the bill and the government could face a large rebellion from its own backbenchers when it comes to a vote in a fortnight’s time.

On Wednesday, Work and Pensions Secretary Liz Kendall told the BBC her “door was always open” to colleagues worried about the bill but that ministers were “firm in our convictions”.

Under the current system too many people were being “written off” instead of being given support to find work, she said.

She also argued that claimant levels were rising to unsustainable levels, and figures released this Tuesday found the number of people on Pips had reached a record high of 3.7m, up from 2.05m in 2019.

Pip is a non-means-tested benefit available to people both in and out of work. It aims to provide support for those with long-term disabilities or who have difficulty doing certain everyday tasks.

In a bid to reassure concerned Labour MPs, Kendall extended the transition period for those losing their personal independence payments from four weeks to 13.

On Wednesday, impact assessments produced by the government estimated that 370,000 existing Pips claimants in England, Wales and Northern Ireland would lose out under the proposed changes, saving £1.7bn by 2029/30.

A further £1.89bn could be saved from a predicted 430,000 drop in the number of potential future claimants.

Another impact assessment, published in March suggested 250,000 people could be pushed into poverty by the cuts – but ministers said the figure didn’t take account of the £1bn it would spend to help the long-term sick and disabled find work.

Overall, the government expects to save £5bn a year by 2030.

As a government whip, Foxcroft would have been expected to persuade reluctant Labour MPs to back the proposed legislation.

The Lewisham North MP said she was quitting because she knew she would “not be able to do the job that is required of me and whip – or indeed vote – for reforms which include cuts to disabled people’s finances”.

She added that she was “incredibly proud to have served as part of the first Labour government in 14 years and hope that ministers will revisit these reforms so that I can continue to support the government in delivering for the people of this country”.

Foxcroft was first elected to her south London constituency in May 2015.

She previously served as a shadow minister for disabled people and in her resignation letter she said that experience taught her that life for disabled people was “even tougher than I had imagined”.

Some of her fellow Labour MPs praised her decision to quit. Connor Naismith, who represents Crewe and Nantwich, said she should be “commended for standing by her principles”.

One Labour MP, who wanted to remain anonymous, told the BBC: “I have heard anyone who breaks the whip won’t be allowed to stand as a Labour MP at the next general election.

“These threats are just making people more angry. We cannot continue to govern in this manner. Quite frankly, if that’s his [the prime minister’s] view he’s lost the plot and is a bully.”