Opening debate on Lords amendments to the Welfare Reform and Pensions Bill, the Social Security Secretary said: "The first thing we are doing in our reforms is to do more for people who need it most. The present system isn't doing enough and this Bill starts to put that right."
Peers last month rejected three disability benefit provisions under the legislation - deleting a clause limiting Incapacity Benefit to people who had been in recent employment; deleting provisions for means-testing IB for people with pension income; and scrapping a clause abolishing the Severe Disability Allowance.
Backbenchers were also likely to support peers who defeated proposals on pension reform, including changes to war widows and the bereavement allowance. Mr Darling said: "We want to do more for the many people disabled from birth and from childhood who have got no chance to work and no chance to make National Insurance Contributions." He agreed the Bill would result in long-term savings, but defended ministers' record on the disabled pledging: "This Government is quite happy to spend more money on disabled people."
But Dr Roger Berry, the Labour MP for Kingswood, moving the rebel amendment, insisted that continued opposition to the proposals was the only way to ensure a Bill all Labour MPs could be proud of. Even with the revised thresholds proposed by Mr Darling, at least 310,000 people unable to work would "lose out under the proposals".
"The Government's proposals will reduce benefits for disabled people unable to work. These are people who have had a work test, have paid their National Insurance contributions and have been declared unfit to work.
"I feel I have no alternative but to vote against the Government's motion and I would urge other Members to do the same."
Tom Clarke, the Labour MP who was a shadow minister for the disabled before the general election, said if the Bill was passed in its present form, "many people would be significantly poorer".
The MP for Coatbridge and Chryston said: "We are talking about people's incomes being in effect taxed at a rate which not one person in the land is normally asked to pay. Those who expected more from our Government do feel deeply betrayed.
"The truth is there is no popular support for this," he told ministers.
Supporting the Government, Sam Galbraith, the MP for Strathkelvin and Bearsden, another former Labour minister and registered disabled himself, said: "This idea that's got around that the contributory principle ... these are all inviolate things that must never be touched cannot any longer be true.
"They may have been true in the early days when the welfare state was set up but they most certainly are not the case today."
Audrey Wise, the Labour MP for Preston said: "I don't believe that there is any disabled person who is in a good position at this point of time in this country."
David Willetts, the Shadow Education Secretary, attacked the means-testing proposals as being "hitherto unknown" in the social security system.
"It is not a means test that takes account of people's incomes in general. It simply takes account of just one form of income."
Earlier, Tony Blair, defended the legislation, stressing since 1979 the number of people on IB had trebled.
"Our reforms won't affect any existing claimants but, if we do not reform the system for the future, we will carry on running up very high bills.
"Meanwhile a lot of people who are severely disabled and who really need the help aren't getting it."
But Charles Kennedy, the Liberal Democrat leader, said "alarm bells" were ringing over the Government's plans to means-test IB because it would "penalise the most vulnerable in society".