Declaring, “hand on heart, I did what I thought was right”, Tony Blair finally paved the way for a Gordon Brown premiership yesterday by announcing that he would leave Downing Street on June 27.

Last night Labour and Westminster were preparing for a new style of more sober leadership once Mr Brown finally secures the job that he has craved all his political life.

Mr Brown launches his campaign today and is certain to be elected at a party conference in London on June 24, three days before Mr Blair departs. It was not clear last night whether the Left had enough votes even to mount a challenge. Either way it will be a coronation because Mr Brown is unbeatable.

Admitting that expectations had not always been realised and apologising “for the times I have fallen short”, Mr Blair was close to tears at the end of the speech in his Sedgefield constituency in which he announced his plan to tender his resignation to the Queen. It is expected that he will make his last appearance as Prime Minister at the weekly Question Time on June 27 and then go to the Palace.

The mood of historic change at the top of the Government was clear last night. Even as Mr Blair was announcing his plans, the logo on the Labour website was changed, removing mentions of “new Labour”.

Officials said that it often changed for elections and denied that it was intended to show the party had already moved away from Mr Blair, but the Tories claimed it was Mr Brown swiftly trying to “airbrush” Mr Blair out of history.

Mr Brown’s launch today is likely to be serious and professional, offering some contrast with the razzmatazz of past new Labour presentations. The Chancellor, who led the tributes to the “unique” achievements of the outgoing Prime Minister, is hoping for a fight, both to define his own programme against the Left and to enliven what would otherwise be a strange one-man contest.

But his determination to use the contest as a springboard for his battle to win Labour a fourth term will be clear as he makes his announcement at a Central London art gallery and then tours five marginal constituencies, not all Labour-held, outside the capital.

He will use his speech to call on Michael Meacher and John McDonnell, the potential left-wing leadership candidates, to debate Labour’s future at a conference this weekend. Aides said that, while his initial meetings will be with party activists, he intends to use the campaign to engage with the public about their concerns.

Jack Straw, Mr Brown’s campaign chief, said: “Gordon will welcome any contest that there is and the opportunity for debate that that will bring.”

Mr Brown will be endorsed by Mr Blair and Blairite ministers including David Miliband and James Purnell today. The new deputy leader to succeed John Prescott will also be announced on June 24. By last night it was clear that Peter Hain, Harriet Harman and Alan Johnson had secured the 44 nominations needed to stand and that Jon Cruddas was close to his target.

Surprisingly Hilary Benn, the International development Secretary, appeared to be struggling for names and the sixth candidate, Hazel Blears, had also not reached the magic total, although she was backed by John Reid, the Home Secretary, last night.

Mr Blair went to his constituency after first telling the Cabinet that he was going. He asked for tributes to wait, until a later day, but Mr Brown intervened at the end of a short meeting to thank him on behalf of all present.

David Cameron, the Conservative leader, said last night that people would look back on ten years of “dashed hopes and big disappointments, of so much promised, so little delivered”.

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