Jeremy Corbyn has urged Theresa May to rethink a "shameful" review of who is eligible for disability benefits.
The Labour leader said the government was planning to over-ride legal rulings and restrict Personal Independence Payments paid to dementia sufferers and those with mental health conditions.
But Mrs May said no existing claimants would see their benefits reduced.
She said the government was clarifying the scope of payments to ensure they were in line with original intentions.
PIPs, which replaced Disability Living Allowance in 2013, are aimed at helping people cope with the extra costs of living with ill health or disability and are made according to the points a person scores in an assessment of their needs.
Recent tribunal rulings have extended the scope of the payments and who is entitled to them - a move which the government has said will add £3.7bn to the benefits bill by 2023 unless it is reviewed.
Clashing over the issue at Prime Minister's Questions, the Labour leader said the move - which was announced last Thursday - was a "nasty decision" and had been "sneaked out" without any warning.
Mr Corbyn said the review would affect 160,000 people with debilitating mental conditions, who had been deemed eligible by the court to get help as they were suffering from "overwhelming psychological distress".
Among those who would lose out, he added, were those recovering from strokes and post-traumatic stress disorder.
"The reality is that this is a shameful decision," he said.
"Can she look at the effects of the decision to over-ride what an independent court has decided and think again?"
"As a society we are judged by how we treat the most vulnerable... can't the prime minister withdraw this nasty decision and support those going through a very difficult time in their lives?"
One tribunal ruling on PIPs in late 2016 found people who needed help take medication or monitor a health condition like diabetes would score the same on the benefits criteria as people who needed help with a therapy such as kidney dialysis
A second ruling said people who struggled to travel independently because of conditions such as anxiety scored the same as someone who was, for example, blind.
Mrs May said the government's review of the eligibility criteria was "not a policy change" and Parliament had been told about it in a written statement.
Rejecting claims of a lack of consultation, she said the Work and Pensions Secretary had informed the Labour MP who chairs the Work and Pensions Committee about the move and the Social Security Advisory Committee could look into the issue.
She said the scope of PIPs had been approved by the Tory-Lib Dem coalition government and approved by Parliament but the legal rulings were fundamentally extending their reach and must be returned to their "original" basis.
"This is not a cut in the amount that is going to be spent on disability benefits and no-one is going to see a reduction in their benefits from what has been previously awarded," she told MPs.
She added: "The way we are dealing with disability benefits payments is to ensure payments are going to those who are most vulnerable.
"What we are doing with regard to PIPs is ensuring", she said, that the intentions and decision of this Parliament "is being put into practice".
The BBC's political editor Laura Kuenssberg said what the government was proposing was "not technically a cut" but would have ramifications for the benefits system.
While some Tory MPs were concerned about the move and ministers were working behind the scenes to allay any fears, she said there was not currently enough "heat" in the issue to force the government to back down at this stage.