Alan Duncan, the shadow prisons minister, said the scheme should be ended only when conditions in prisons were right, rather than for political reasons. "It is right that the early release scheme should go, but only when the correct measures have been put in place to allow this to happen," he said.

"Labour is seeking deliberately to leave us with a poisoned pill by claiming credit for doing this in March, while knowing that it will leave us, after the election, with a crisis in June."