Jacqui Smith, Hazel Blears – these were resignations that Gordon Brown might just have shrugged off. The departure from the cabinet of James Purnell is of an altogether different order. Mr Purnell, until Thursday night the work and pensions secretary, was one of the brightest and the best of the government’s ministers. He is a politician with a worldview, and to his admirers at Westminster, a future Labour leader and prime minister. His decision to quit now describes the intensity of the crisis engulfing Mr Brown’s premiership.

The prime minister’s allies were on Thursday night attempting to dismiss Mr Purnell as a Blairite ultra set on refighting old battles between Mr Brown and his predecessor. That is too glib a description of a formidable young politician. There is no doubt about Mr Purnell’s New Labour credentials. He has long been in the forefront of those ministers who argued that unless the government remained committed to the reforms put in train by Mr Blair it would surrender the centre ground to David Cameron’s Conservatives. In that respect, his impatience with Mr Brown has been an open secret.