The US and Germany are drawing battle lines on the streets of London in a row over the congestion charge.
Embassy staff have not paid the charge since July
Staff at the US and German embassies have not paid the charge since July.
The two parties are claiming diplomatic immunity under the 1961 Vienna Convention, which gives protection against paying taxes.
But London's mayor, Ken Livingstone, says the £8 toll for driving into central London is not a tax but a charge for a service.
A Transport for London spokesman could not give a figure on how much is owed but said the embassy had 100 registered cars in the fleet scheme - although not all of them are subject to the congestion charge.
Fines can reach £150 per car, per day for payments due for more than 28 days, which could mean fines of up to £15,000 a day.
A US embassy spokeswoman said: "We consider it a tax, and it is the view of the United States government that all direct taxes on diplomats and diplomatic operations, including this one, are prohibited by the Vienna Convention."
A German Embassy spokesman said: "We received a memo from Berlin several months ago which explained that the congestion charge is considered to be a tax and therefore diplomats are exempt."
He said it was only diplomatic staff with embassy vehicles who were not paying. British staff with private cars pay.
But a spokesman for the mayor said: "The congestion charge is not a tax. It is a charge for a service.
"All staff at the American embassy should pay the congestion charge, in the same way as British officials pay road tolls in the United States.
"The congestion charging scheme gives no privileges to any VIPs, including the mayor, MPs, (London) Assembly Members or councillors, therefore we believe diplomats should pay."
Diplomatic staff from Russia, Japan and Spain confirmed they all paid the charge.
The congestion charge was introduced in February 2003 and has been credited with reducing traffic in the controlled zone by about 30%. Journey times have fallen by 14%.
From February 2007, the zone will be extended to include parts of west London.
The charge has faced criticism, especially from business groups which claim its has affected trade.