The Prime Minister will use his annual Mansion House foreign policy speech to try to quash speculation that he values transatlantic ties less than his predecessor, Tony Blair.

He will also signal support for America's tough stance against Iran - insisting now is the time to "ratchet up" diplomatic pressure, and again refusing to rule out support for military strikes.

A Downing Street source said Mr Brown was determined that there would be "no misunderstandings" about his enthusiasm for the special relationship.

Senior figures in Washington have become increasingly concerned that the new premier has been trying to distance himself from President Bush.

Fears were intensified over the summer by comments from foreign office minister Lord Malloch Brown that the two countries would no longer be "joined at the hip".

There was also alarm over moves to "draw down" UK troops in Iraq.

Mr Brown is expected to tell the Guildhall event that the arrival of new leaders in France and Germany means that Europe and the US have the best opportunity for closer co-operation for "many years".

The Western powers should be working together to carry out vital reforms on international organisations such as the UN, he will say.