Tony Blair has considered reform of incapacity benefit as unfinished business ever since a Labour backbench rebellion in 1999 forced the government to water down a previous attempt to overhaul a key part of the welfare state.

There are 2.5m incapacity benefit claimants, equivalent to 7.6 per cent of the working age population. IB costs the Treasury £7bn a year, rising to £13bn when other disability benefits are included.