US diplomats stopped paying the charge in July 2005

It said the embassy has failed to pay the charge since July 2005 making it "by far the worst offender in terms of unpaid charges and penalties".

Mayor Ken Livingstone has renewed his call for the US government to "respect British law and pay the charge".

The embassy said the charge is a local tax which, under the Vienna Convention, does not apply to foreign diplomats.

Mr Livingstone, who has previously made clear his disapproval of the embassy's position, said the charge was not a tax.

He said: "They are entitled to their opinion but it is for the British authorities to decide what is a tax and what is not a tax in the UK."

TOP DIPLOMATIC OFFENDERS US - �1,016,200 Angola - �543,300 Nigeria - �504,120 Sudan - �449,220 Tanzania - �233,630 South Africa - �214,640 Kenya - �208,610 source: London mayor's office, October 2006

Mr Livingstone said the congestion charge was a charge for a service - reduced congestion.

"The US government rightly expects international diplomats living in Washington to respect US law," he said.

"We are entitled to expect US diplomats living in London to respect British law."

He said British diplomats in the US pay tolls and charges and urged American diplomats to do the same.

An embassy spokesman said: "The US is not alone in its stance, as some 50 other embassies in London also refuse to pay the tax.

"US diplomats do pay parking fees, road tolls, and traffic violation fines."

The mayor's office has said there were 19 embassies who consistently failed to pay the congestion charge.