Gordon Brown’s long and impatient 13-year wait to become Prime Minister ends today as the man who has held him up for all that time bows out of domestic politics.

Mr Brown will take over from Tony Blair at the age of 56, boosted by the shock defection to Labour last night of the Conservative MP Quentin Davies.

Within hours Mr Blair will announce that he is leaving Parliament to take up a new post as an envoy to the Middle East.

Mr Davies, a pro-European Tory, left his party with a withering attack on David Cameron, telling him that under his leadership the Conservatives have ceased “to believe in anything, or to stand for anything”.

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It was a huge coup for Mr Brown. Mr Davies added in a letter delivered to the Tory leader as he announced his switch to Labour: “Although you have many positive qualities, you have three — super- ficiality, unreliability and an apparent lack of any clear convictions — which in my view ought to exclude you from the position of national leadership.”

On a day of history and drama in Whitehall, Mr Blair will make his last appearance in the Commons at Prime Minister’s Questions before going to Buckingham Palace to resign.

Shortly afterwards Mr Brown will be called in to form a government. Then he will go to No 10 as Britain’s 52nd Prime Minister to prepare to announce members of his administration and talk to foreign leaders.

Mr Blair will go to his Sedgefield constituency to announce that he is standing down as an MP, prompting a by-election late in July.

The arrival of Mr Davies as Labour’s newest MP stunned Westminster and was a big present for Mr Brown on the eve of him finally taking the job that he has cherished all his political life.

The Blair-Brown partnership that has dominated politics for so long will be suddenly over and Mr Brown will be on his own. The Granita deal — named after the Islington restaurant where they mapped out their futures in 1994 — will have been belatedly fulfilled.

Mr Blair will be named today as the special envoy for the international diplomatic Quartet on the Middle East. His appointment was discussed by representatives of the Quartet — the US, the United Nations, the European Union and Russia — at talks yesterday in Jerusalem. He will be entitled to a pension of around £63,000, half his salary, and an £87,000 allowance to help him to pay for his continuing public duties as a former prime minister.

Mr Brown saw Mr Davies several times in his Treasury office as the MP considered switching to Labour. But the talks were at the instigation of Mr Davies and he was said to have demanded nothing in the way of ministerial office in return. Though not well known, he is an important scalp for Mr Brown. Mr Davies said that the Tory party had no bedrock. “It exists on shifting sands. A sense of mission has been replaced by a PR agenda.”

Mr Davies said that he had found himself increasingly “naturally in agreement” with the Labour Party. He praised Mr Brown as “a leader I have always greatly admired, who I believe is entirely straight- forward, and who has a towering record, and a clear vision for the future of our country which I fully share”.

He told Mr Cameron: “It is fair to say that you have so far made a shambles of your foreign policy, and that would be a great handicap to you — and, more seriously, to the country — if you ever came to power.”

A delighted Mr Brown said: “Quentin Davies is a senior parliamentarian and he commands respect on all sides for his expertise and his dedication to public service, and I welcome him to the new Labour Party.”

The Tory peer Lord Tebbit said: “This defection will raise the average standard of members on the Conservative side and lower it on the Labour side.”

Last year, Mr Davies called Mr Cameron’s decision to vote for an immediate inquiry into the Iraq war “absolutely crazy”.

He also said that the party risked looking like “dishonest double-glazing merchants” over plans to withdraw from the European People’s Party group in the European Parliament. The 63-year-old is a former diplomat and has been Shadow Northern Ireland Secretary and Shadow Defence Secretary. He became an MP in 1987.

Mr Cameron wrote back to Mr Davies: “Your decision does not come as a surprise to me. I am sorry that you feel unable to be part of today’s Conservative Party and join us in campaigning on what matters to people.

“The big dividing line in British politics is between Labour’s approach of top-down state control and the Conservative vision of pushing power outwards and downwards from central government, trusting people and sharing responsibil-ity with them. You have made your choice and the British people will make theirs.”

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