James Purnell’s resignation has, at a stroke, tilted the odds against Gordon Brown’s survival as Prime Minister. Both the timing and manner of his departure are devastating. The future of Mr Brown is now out in the open. There is no longer any code, any disingenuous declarations of loyalty. It is a battle for power.

The departures from the Cabinet of Jacqui Smith and, then, Hazel Blears were serious enough, but both could be explained as responses to the MPs’ expenses row. Mr Purnell is a different class. He has gone explicitly because he believes Mr Brown should resign in order to give Labour “a fighting chance”.

Mr Purnell is also one of the most successful younger members of the Cabinet, upholding the flag of public services reform on welfare. He is a man with a future who has shown overnight that he has both courage and political weight. He has not been plotting, did not consult Blairite allies such as Tony Blair himself or Lord Mandelson, and he has ruled himself out as a possible candidate for the succession.

In the immediate future, Mr Purnell’s departure threatens Mr Brown’s attempt to relaunch his premiership with a reshuffle. There are already three Cabinet vacancies, and two so far among middle-ranking ministers. One senior minister told me today that he expected more resignations. And if other ministers refuse to serve or be moved to other positions. Mr Brown may find it impossible to construct a convincing reshuffle.

The key is credibility. Of course, a prime minister can always find ambitious politicians willing to fill vacant places, and governments have survived multiple departures for a time, though seldom for very long. Moreover, the wear and tear of 12 years in office, and the change of premiership two years ago, have resulted in a huge turnover of ministers. The ranks of the fallen are pretty impressive, more so than the ranks of current ministers.

Labour’s bench of replacements is now looking pretty thin. The middle ranks of the Government are uninspiring. Among the stronger candidates, Margaret Beckett, that indestructible safe pair of hands, and Liam Byrne, who both attend Cabinet meetings, are likely to be promoted to full Cabinet status. Watch out also for Pat McFadden, who has been dealing with the Post Office, and John Healey, the Local Government Minister and a long-term Brownite ally.

Apart from Mrs Beckett, there is a severe shortage of suitable female candidates. Otherwise, it is a matter of scratching heads about x or y undistinguished, and unknown, minister of state.

The risk for Mr Brown is that his Cabinet will be dominated by close allies, such as Ed Balls, Yvette Cooper and Baroness Vadera (also a candidate for promotion). All three have considerable strengths as ministers and advisers but such a team will be depicted as Mr Brown’s cronies, and very vulnerable to criticism from the Conservatives.

This would be all too reminiscent of the final days of the Balfour Administration in 1903-05 when, after multiple resignations, the Cabinet was so full of the Prime Minister’s relatives from the Cecil family that it was known as the Hotel Cecil.

And remember, this is all before the impact of today’s elections. This morning will come the results of the local council elections and then, on Sunday evening, the European elections. Both are expected to be very bad for Labour.

Mr Purnell’s departure makes the situation even more unstable and harder for Mr Brown to control. The tide of news is running strongly against him. Mr Brown has virtually no cards left, certainly no longer any obviously winning ones.

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