The Conservatives have promised to cut the number of incapacity benefit claimants in Britain by 200,000.
The Tory incapacity reforms involve a new testing system
Tory leader David Cameron said too many people were able to work, but did not.
The party is pledging to bring in more regular tests for people claiming the benefit - which costs the taxpayer £12.5bn every year.
Some disability groups have criticised the plans, saying that levels of fraud are low. The Labour Party says the Tories have copied its ideas.
Under the Conservative plans, to be announced in full on Tuesday, all existing and new Incapacity Benefit (IB) claimants would have to attend an "in-depth assessment" to evaluate their needs and capabilities and decide if they are able to work.
Those deemed fit to work would be taken off IB and put onto Jobseekers' Allowance.
This would mean a £20-a-week cut in benefits and a requirement to seek work immediately. The Tories are promising that those who have the "potential" to be able to work would be referred to specialised welfare-to-work providers who would help them prepare them for jobs and would be paid by results.
People with permanent disabilities which make it impossible to work would continue to receive "unconditional" support, but those with non-permanent conditions would be subject to regular checks.
Writing in the News of the World, Mr Cameron said he was particularly shocked by the rising number of younger IB claimants.
'Culture of despair'
An estimated 500,000 people are now claiming IB, while claims from under-25s have risen by more than half over the last 10 years.
2.64m people claim IB
It costs the country £12.65bn every year
More than half of IB claimants have been on benefit for 5 years or more
The number of young people claiming IB is up by a half
Mr Cameron said: "I don't believe that there are nearly half a million young people in Britain with a disability which prevents them from doing any work at all.
"What we have is a culture of despair, where kids grow up without any idea that for our society to function everyone has to pull their weight if they can."
Speaking on the BBC's Andrew Marr Show, Prime Minister Gordon Brown said: government's own proposals were "far more revolutionary" than those put forward by the Conservatives.
He said: "Today the issue is people don't have the skills, even when there are 600,000 vacancies in the economy."
Mr Brown also said: "The next stage is not what the Conservatives are talking about but giving people the skills to get into work."
Work and Pensions Secretary Peter Hain said of the Tory proposals: "They are plagiarising plans already announced by us before Christmas and seeking to present them as their own.
"The Tories have no credible plan to fund an expansion in 'welfare to work'.
Liberal Democrat work and pensions spokesman Danny Alexander said: "Labour and the Conservatives are united in being more interested in harsh rhetoric about benefits than in serious action to help people find jobs.
"The only sustainable way to reduce incapacity benefit is to get people into work."
The Conservatives are the latest party to focus on the large number of IB claimants, estimated to have nearly quadrupled since the 1970s.
The figure has fallen slightly over recent years to about 2.6m people - but experts are still puzzled at the overall trend, given the UK's rising health and living standards.
There have also been a number of high-profile cases of apparently healthy people fraudulently claiming the benefit, while secretly holding down physically demanding jobs.
The government has already pledged to introduce tougher tests for those claiming IB from this autumn. It says it expects to cut the number of claimants by 20,000 each year.
But the tests will initially apply only to new claimants, while the Conservatives say they are determined to make all claimants - including existing ones - prove they qualify for the £81-a-week benefit.