Incapacity benefit crackdown begins after pilot scheme
The one-and-a-half million people who claim incapacity benefit will start to receive letters this week asking them to be tested on their ability to work.
The new assessments are part of government plans to reduce the number of long-term claimants in a rolling programme through to 2014.
Almost 30% of those who took the test during pilot schemes in Burnley and Aberdeen were declared fit to work.
However, disability charities say many of the assessments are unfair.
But Work and Pensions minister Chris Grayling said: "My message to people who are worried about this process is that this is all about helping those who can return to work.
"It's not about forcing people to return to work, but unless we do the assessments, unless we identify who has that potential, we'll never be able to offer that help."
During the pilot scheme, a further 39% were assessed as able to work but needing the right support to do so.
Under the initiative, once claimants have had a work capability assessment they will be placed in one of three groups.
Those immediately fit for work will be put on jobseeker's allowance.
Those deemed unable to work because of sickness or disability will be entitled to the highest rate of employment support allowance, and will not be expected to look for work.
A middle section - those judged unfit to work but for whom an eventual return to the workforce is realistic - will be placed in a "work-related activity group".
They will be expected to take steps to prepare themselves for a return to employment.
Early indications showed 70% of incapacity benefit claimants had the potential to work.
But Mr Grayling told the BBC there were "absolutely no targets attached to this programme".
"Of course, if we get people back into the workplace it will save money for the taxpayer in the end, but we do not have a financial target."
He also said the assessments would only be one factor taken into account when determining fitness to work, and even greater emphasis would be placed on expert evidence from a person's GP or consultant.
The fitness-to-work test was changed last year after widespread criticism.
Charities expressed concern that it unduly focused on an individual's physical capacity and ignored other factors such as mental health issues.
They also warned that people were not being given enough help to prepare for the test.
But Richard Hawkes, chief executive of disability charity Scope, told the BBC that changes made since then had not gone far enough.
"This test is a very blunt medical questionnaire where you sit across the room from somebody you've never met before," he said.
"It just doesn't take into consideration things like fluctuating impairments, or things like ME [chronic fatigue syndrome] where you might not be able to do things over a sustained period of time."
This was supported by an independent review of the system in November calling for a "more fair and effective" process.