Mr Blair and Mr Bush will discuss the forthcoming G8 summit, the latest world trade round, climate change and the crisis in Darfur.

The visit is part of the Prime Minister's "business as usual" approach to his final weeks in power, representing the UK at international gatherings where crucial decisions will be taken.

Mr Blair will have dinner with the President on Wednesday night, followed by a working session in the Oval Office on Thursday morning.

The two leaders will then host a joint press conference.

On Thursday night there will be a reception at the British Embassy to mark the restoration of devolved government in Northern Ireland, with inevitable speculation that former President Bill Clinton may be among the guests.

The PM's official spokesman said: "The World Trade Organisation talks will be high on the agenda because we are approaching crunch point. The run-up to the G8 will be important, particularly on climate change."

The G8 summit takes place in Germany at the beginning of next month, and Mr Blair is keen to pursue his Gleneagles agenda of green issues and aid for Africa.

His spokesman added that Mr Blair believed the world should also send a "very clear message" on Darfur telling the Sudanese government to come into line with international, UN-backed demands.

Iraq and the Middle East peace process will feature in an unusually-long session of talks, which will be seen as the last protracted dialogue between Mr Blair and Mr Bush, whose alliance after the terror attack of 9/11 led to the invasion of Iraq.