Sir David is favourite for plum post in Washington

Sir David Manning, the foreign policy adviser to Tony Blair at Downing Street, is set to become Britain's next ambassador to the United States.

A career diplomat, Sir David has been lined up to replace Sir Christopher Meyer, who is leaving the plum Washington posting next February to become chairman of the Press Complaints Commission.

Sir David, 52, has forged close links with the Bush administration since joining Number 10 from the Foreign Office in September last year. He is in regular contract with his opposite number Condoleezza Rice, the US National Security Adviser.

Described by Mr Blair's aides as a very clear thinker and a pivotal member of his "Rolls-Royce team" of advisers, Sir David has worked closely with the Prime Minister during a period in which foreign affairs have taken up much of his time after the terrorist attacks in the United States last September.

He played a crucial role in ensuring that the Prime Minister and President Bush stood "shoulder to shoulder" in the campaign against terrorism.

Although Whitehall sources insisted the Prime Minister had not made a final decision on sending Sir David to Washington, one insider said last night: "It's a done deal."

Mr Blair is said by close colleagues to be reluctant to lose Sir David from his Downing Street inner circle, but is believed to have concluded that he is the best man for the Washington post. One aide said: "He's a good thinker, hard worker, absolutely dependable and with a strong personality – all the qualities you need for Washington."

Sir David's claims have been pressed strongly by the Foreign Office, which is keen to see the position filled by a career diplomat rather than an "outsider" such as Peter Mandelson, the former cabinet minister. Downing Street dismissed speculation at the weekend that he could get the job and Mr Mandelson said he did not want it.