More than 13,000 disabled people are to receive backdated benefit payments after the government accepted a court ruling over universal credit.
Those being paid are people who moved from a benefit called severe disability premium to universal credit, which rolls six payments into one.
The Department for Work and Pensions agreed to the back payments after a High Court ruling found two disabled men had been discriminated against.
MPs dismissed the "11th hour" move.
Universal credit, which is being introduced in stages across the UK, combines six separate benefits for working age people into one payment.
Supporters of the welfare reform say it helps simplify the old complicated benefits system.
But since its introduction in 2013, it has been accused of making things harder for people receiving it.
Addressing the House of Commons on Monday, Work and Pensions Secretary Amber Rudd said claimants who had been entitled to the severe disability premium would be given "ongoing transitional payments" as they moved across to universal credit.
She said people who had already moved to the new system would be eligible for backdated payments of the disability benefit.
Ms Rudd said claimants would receive up to £405 per month alongside the universal credit benefit, instead of the previous maximum figure of £360.
She said by 2024-25 about 45,000 of the "most vulnerable" claimants would benefit from the support package worth £600m.
But shadow work and pensions secretary Margaret Greenwood said Ms Rudd's approach was "deeply controversial", adding: "You have left it until the 11th hour to bring these regulations to Parliament."
SNP employment spokesman Chris Stephens added the short notice of the changes was "disrespectful".
The BBC's social affairs correspondent Michael Buchanan said the DWP had been forced to agree to make the back payments after the government was successfully sued by two disabled men in June 2018.
The men had lost their severe disability premium top-up payment when they were moved onto universal credit. The High Court found they had been discriminated against.
Ms Rudd said the next stage in the rollout of universal credit later this month will see her department encouraging thousands of people in Harrogate, North Yorkshire, to apply.
MSPs in Scotland have called for the Yorkshire rollout to be stopped over fears about the impact it could have.