From The Guardian:
James Purnell, the work and pensions secretary, tonight dealt a monumental blow to Gordon Brown’s chances of holding onto office when he dramatically announced he was quitting the cabinet and asking Brown “to stand aside to give Labour a fighting chance of winning the next election”.
His statement, effectively declaring Brown as unelectable, will further weaken the prime minister’s waning authority and takes the challenge to his leadership to a new dangerous level. Purnell made his sensational move after polls closed in the local and European elections, informing Brown by phone tonight.
The prime minister had no inkling that Purnell was going to quit, since the work and pensions secretary shrouded his move in secrecy in order to prevent Number 10 mounting a pre-emptive strike against him, or seeking to challenge his motives.