The rising benefits bill is “devastating for public finances” and has “wreaked a terrible human cost”, Keir Starmer has said as he defended the government’s drastic changes to the welfare system.

Writing in the Times, the prime minister said “the facts are shocking”, noting one in eight young people were not in education, employment or training and 2.8 million working-age people were out of work because of long-term sickness.

Starmer said the picture was a “damning indictment of the Conservative record”. Reflecting on the former chancellor George Osborne’s criticism of Margaret Thatcher when he claimed she had let people “languish” on benefits, Starmer said Osborne had done the same thing.

Defending his government’s measures, which experts say will push more people into poverty, Starmer wrote: “Young people shut out of the labour market at a formative age. People with complex long-term conditions, written off by a single assessment. People who want to return to work, yet can’t access the support they need. All this is happening at scale and it is indefensible. An affront to the values of our country and Labour’s history.”

While most Labour MPs accept the figures are unsustainable, many have struggled to digest the government’s proposed changes to eligibility criteria for the personal independence payment (Pip), a benefit aimed at helping people with disability or long-term illness deal with increased living costs, and its proposal to delay access to the health element of universal credit until a young person is 22.

The social security minister, Stephen Timms, said people with conditions such as anxiety would still be able to claim Pip if the effect on their wellbeing was deemed severe enough. But the proposed changes mean people will need to score a minimum of four points in at least one activity to qualify for the “daily living element” from November 2026, which is a high threshold.

About a million people will completely lose their right to Pip benefits, according to the Resolution Foundation, and the government has refused to release its impact assessment until next week.

Labour MPs were invited to a briefing with Timms on Tuesday night to get more insight on the government’s changes and share their concerns. Only a dozen MPs turned up, despite many more being expected to vote against the governments plans. “They scheduled it at the last minute as the children’s wellbeing bill was being debated and voted on,” a Labour insider said.

“They don’t actually care if anyone has any dissent to vocalise; no one thinks it’ll make a difference at this stage. This is an ideological pursuit. If they wanted to find money elsewhere they could.”

The backlash was in full swing on Tuesday as Kendall took almost 100 questions from MPs, many of them from Labour, as she outlined the case for her green paper.

Debbie Abrahams, the chair of the Commons work and pensions committee, said: “There are alternative, more compassionate ways to balance the books rather than on the back of sick and disabled people.”

She told the Commons the government’s plans to cut £5bn from benefit support was the largest reduction in welfare support since 2015.

The Disability Benefits Consortium, an umbrella body representing more than 100 charities and organisations, said the “devastating cuts will push more disabled people into poverty and worsen people’s health”.

On Wednesday, Timms told Sky News the government was considering extending disability living allowance to parents after the death of a child they had given up work to care for “just to support parents during what otherwise is obviously a terribly, terribly difficult time for them”.

The minister did not rule out further benefit cuts in the future, as he told Times Radio: “Who knows what will happen in the next five years?”