The Government’s own welfare experts have attacked a controversial decision to deny disability benefits to 160,000 vulnerable people – and urged ministers to shelve it.

The changes to Personal Independence Payments (PIPs) – affecting the mentally ill - should be delayed until they have been properly tested and “clearly understood”, ministers are told.

The experts also warn it is “not clear” how assessors will interpret the changes – raising the danger that claimants will not be “consistently treated”.

And they dispute ministers’ claims that emergency legislation must be rushed through tomorrow, suggesting a feared leap in costs has been over-hyped.

The damning conclusions sparked angry exchanges in the Commons, with some Tory MPs joining Labour and the Liberal Democrats in criticising the impact of PIPs on the vulnerable.

But Damian Green, the Work and Pensions Secretary, refused to allow MPs to vote on the changes – insisting that was “above my pay grade”.

Mr Green also acknowledged “a handful of people” could now have their PIP payments cut, having been awarded higher sums in the last few months.

Debbie Abrahams, Labour’s Work and Pensions spokeswoman, said that contradicted repeated assurances – including by Theresa May – that no disabled people would lose money, with only new claimants affected.

And she said: “The Government’s decision to change the law on PIP is a clear example of the way people with mental health conditions are not given equal treatment.”

The row follows the Government’s decision to tighten the criteria for PIPs, after a tribunal ruled they should also cover conditions including epilepsy, diabetes and dementia.

The ruling would cost at least £3.7bn over the next five years, money which should go to “really disabled people who need it”, one minister said – before later apologising.

The tribunal also said that claimants who needed support to take medication should be assessed in the same way as those managing therapies such as dialysis at home.

When the controversy first blew up last month, some Conservative MPs threatened a revolt, urging ministers to stand by the tribunal’s ruling.

But the Department for Work and Pensions (DWP) used what is called a “negative resolution” to push through the change, denying the Commons a vote.

As a result, it comes into force tomorrow – despite the heads of 32 charities warning it will leave many disabled people without vital financial support.

In its report, the Social Security Advisory Committee (SSAC) said it was “particularly concerned” that overturning the tribunal’s ruling will cause confusion.

“It is not clear how tribunals, decision makers, or health care professionals conducting assessments will respond to changes in descriptors to exclude ‘psychological distress’,” it warned.

The SSAC urged the DWP to consider “testing the proposed changes with health care professionals and decision makers to ensure the policy intent behind the regulation is clearly understood”.

And it concluded: “The department should both (a) consult more widely with representative bodies and health care professionals; and (b) improve the estimate of likely impact before the changes are introduced.”

Answering an urgent question, Mr Green insisted the SSAC was “not challenging the decision” to tighten the criteria for PIP.

But he added: “We think there may be a handful of people whose appeals have gone through the courts in this very, very small period.”