Britain's special relationship with the United States has been strained in recent months by that country's willingness to go it alone on Iraq and by its withdrawal from international treaties on the environment.

Now Washington is about to test the relationship further with a challenge to the mayor of London's plan to charge motorists £5 to enter the centre of the capital.

The US wants its 700 staff at the London embassy to be exempted, saying that if they were forced to pay it would be a breach of the Vienna convention's rule that diplomats abroad should not be taxed; if they do have to pay, the US will consider imposing an equivalent charge on British diplomats in the States.

A spokesman for the embassy said: "We have tried in a variety of ways, in concert with other nations, to find a way out, but we have been rebuffed at every turn.

"For us it is a matter of principle. As a consequence we are considering what sort of reciprocal arrangements will be possible in Washington."

The mayor, Ken Livingstone, believes the scheme is the best way to reduce traffic jams that have slowed the average speed in central London to 10mph.

Mr Livingstone told the BBC: "This isn't a tax, and that's quite clear; it is road pricing. What we're simply saying is if you want to do anything in central London, it costs more than it does in Bromley.

"Everything costs more, the only thing that isn't reflecting that is road space. So what we end up with [at present] is taxing it by queueing, I mean it's almost like the old Stalinist regime. Basically half the time a driver is in central London they are stalled in a jam."

The Foreign Office yesterday appeared to come down on the side of the mayor, saying there was no reason diplomats should be exempted. It said: "We carefully considered whether the UK is under an obligation to exempt diplomatic missions from charges under that scheme, and our view is that there are no legal grounds to do so."