Robert Maclennan, the Liberal Democrat spokesman, protested about 'double dealing' by the Government in a letter to Kenneth Clarke, the Home Secretary, after it was confirmed that the complement at four prisons had been reduced.
Kenneth Baker, the former Home Secretary, said privatisation of the escort service would lead to prison officers being released for other duties for which they had been trained.
But the Government agency responsible for the prisons confirmed that Hull, Leicester and Lincoln prisons had each reduced its complement of officers by 20 and Moorland prison by 10.
Mr Maclennan said: 'This will appear to many to be double dealing by the Home Secretary. The staff in these prisons have now been forced into working more unsocial hours, stretching the regimes to breaking point.'
Group 4 has been criticised after a series of break-outs by prisoners in its custody.
The prisons agency denied that the award of the contract was connected to the reduction in the complement of officers at the four prisons.