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.