David Miliband admitted last night that he considered following James Purnell out of the Cabinet, as Hazel Blears apologised for leaving early.

And Gordon Brown’s hopes of a return to stability were disturbed further when his close ally Lord Mandelson gave him a warning and some public advice on leadership.

Lord Mandelson wrote off Labour rebels as a “small group who keep coming back”, but Mr Miliband, the Foreign Secretary, confirmed that he had to think before staying in the Cabinet.

“I’d made my decision on Thursday. Sometimes you can make your decisions with great planning and calculation and sometimes you have to make them rather more quickly,” Mr Miliband told The Guardian. Labour had received an “electric shock” at the polls, he said. Calling on the party to deny David Cameron a landslide, he came out in favour of the alternative vote system for the first time.

Meanwhile, Lord Mandelson, the new First Secretary of State, outlined political leadership to The Daily Telegraph. “You don’t have to be too grey or serious the whole time,” he said. “You can do your work and enjoy it at the same time and include people along the way.”

Lord Mandelson said he wouldn’t lose any sleep over Labour plotters as the left-wing Campaign Group said that it was considering standing under its own manifesto at the next election.

Hazel Blears tried to salvage her political career yesterday with an apology for the “stupid” manner of her resignation on the eve of last week’s local and European elections.

The former Communities Secretary admitted that it had helped to drive down Labour’s vote to a record low, something she would “regret for ever”.

Interviewed by the Manchester Evening News, she said her mockery of Mr Brown’s YouTube broadcast had been “thoughtless and quite cruel”, and she apologised for the “Rocking the boat” brooch that she had worn when quitting.

Weakened by controversies over her expenses and criticised by Mr Brown as “totally unacceptable”, Ms Blears now faces deselection over the timing of her resignation. It is understood that three branches of Ms Blears’s Salford constituency Labour Party are putting forward motions of no confidence next week.

A Labour MEP who narrowly lost his seat to the BNP said yesterday that Ms Blears was to blame. Richard Corbett said: “She threw a tantrum because she knew she would be dropped in the Cabinet reshuffle.”

Ms Blears said yesterday: “I genuinely thought I could go without it sparking off this huge firestorm,” denying that she had hoped to trigger other resignations. “In hindsight . . . I should have waited until after the election.”

Mr Brown sought to consolidate his position with a special meeting of his new Cabinet yesterday. The three-hour session was called to finalise details of a “mini-manifesto” that is to be launched next week.

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