London's mayor is promising to stand his ground and make US embassy staff pay the congestion charge.
Embassy staff have not paid the charge since July
Ken Livingstone said it was "really outrageous" that they had refused to pay the £8-a-day charge since July.
The US is claiming diplomatic immunity under the 1961 Vienna Convention, which gives protection against paying taxes, and says several embassies do not pay.
But Mr Livingstone said it was a charge for a service, not a tax, and vowed to take them to court to make them pay.
He said: "Since this new ambassador took over in July they have not paid.
"When British troops are putting their lives on the line for American foreign policy, it would be quite nice if they paid the congestion charge.
"We will find a way of getting them into court either here or in America. We are not going to have them skive out of their responsibilities."
Rick Roberts, of the US embassy, said local London staff paid the charge, but accredited embassy officials do not.
He said he believed other embassies also refused to pay, and said it had nothing to do with the arrival of new ambassador, Robert Tuttle.
"Everyone paid it while we were discussing the matter prior to July 2005 but talks were getting nowhere and we took unilateral action," he said.
"We've made our position clear - this is a congestion tax.
"We don't impose tax on British envoys in Washington, and we don't expect tax to be imposed on US envoys in London."