The first phase of the Government's "radical" welfare reform programme is starting with benefit claimants being reassessed for their ability to work.
The move comes as new figures show that almost £135 billion has been spent over the past 10 years keeping two million people "on the sick".
Long-term incapacity benefit claimants in Burnley and Aberdeen will be the first across the country to undergo a new test - the Work Capability Assessment - to see if they are fit for work.
Ministers said the reassessment is designed to end the one-size-fits-all approach to those with health conditions and disabilities which has led to 2.1 million people being "trapped" on benefits.
They said the most severely disabled and people who are terminally ill will not be expected to look for work and will get extra help through Employment and Support Allowance. But those people who are reassessed and found fit for work will move on to Jobseekers Allowance and will be expected to look for work.
Employment minister Chris Grayling said: "It's nothing short of a scandal that so many people were simply cast aside to a lifetime on benefits, wasting their talents and potential and costing the taxpayer almost £135 billion. While some of these people will be genuinely too sick to work, there will be others who through no fault of their own were told by the state that they were better off on the sick and then left behind - this stops now."
Mental health charity Mind called for a revision of the test before it is rolled out to over 1.5 million claimants nationwide. Sophie Corlett, Mind's director of external relations, said: "The benefit test being used in the pilots starting today has a fundamental problem when it comes to people with mental health problems - it does not do what it's set up for, which is to distinguish accurately which people can work and which people can't."
She added: "The Government wants to toughen up on benefit claimants while supporting those in genuine need - this system doesn't identify those in genuine need."
Speaking on the BBC Radio 4 Today programme, Mr Grayling denied there was a "target" to declare 23% of incapacity claimants well enough to get jobs.
He also said the test could be changed if it was not giving an accurate picture of individuals' conditions, saying: "If we need to make modifications to the test to get this right then we will do it."