The PM's Work and Pensions Secretary, Liz Kendall, announced £5billion in cuts claiming the social security system was "failing the very people it is supposed to help and holding our country back".
Labour ministers will aim to slash welfare spending by £5billion a year by 2030 by making it harder to get disability benefits and restricting payouts to young people.
The reduction in spending will also have a "detrimental" impact on the annual block grant awarded to the Scottish Government.
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The cuts prompted one former Scottish Labour MSP to announce he has resigned from the party after 35 years as a member.
Neil Findlay accused the UK Government of trying to "punish and stigmatise the weak, poor and the vulnerable" with reforms to the benefits system.
In a scathing letter to Starmer, the ex-Lothian MSP said: "At a time when more people are going hungry, fuel bills are soaring and the cost of living is leaving working class families unable to afford basics, a Labour Government should be going after the billions lost in corporate tax fraud and avoidance, it should be making those companies that pollute our environment pay and it should be introducing a wealth tax on the super rich.
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"But instead you choose to punish and stigmatise the weak, poor and the vulnerable." Kendall insisted the UK Government was taking "decisive action to fix the broken benefits system" in a bid to have a "more pro-active, prowork system for those who can work".
Eligibility for disability payments will be restricted to those with the greatest need and the test to qualify for the extra Universal Credit payment for health conditions will be replaced.
The health-related top-ups will also be frozen for current claimants and nearly halved for new applicants.
In Scotland, the Adult Disability Payment is in the process of replacing PIP but around 80,000 Scots still receive the old benefit.
ADP is devolved and paid through Social Security Scotland.
Shirley-Anne Somerville, the SNP Social Justice Secretary, said she did not yet know what the financial implications of Kendall's announcement would be on the Scottish Budget.
Somerville said: "The announcements are absolutely devastating for disabled people right across the UK.
I would strongly urge the UK Government to reconsider.
"The reaction from disabled people shows the strength of anger - and real concern and desperate worry they have now placed on some of the most vulnerable in our society."
She added: "If the UK Government does change Personal Independence Payments, they would reduce the amount of money that the Scottish Government gets - that's the amount of money we would spend on Social Security.
"Clearly, any changes the UK Government makes o will have a detrimental effect on the d Scottish Govern...