Ken Livingstone will be thumbing through his thesaurus in search of new adjectives. The US embassy in London - whose head has been described by the London mayor as a "chiselling little crook" - now owes more than £2m in unpaid congestion charge payments and other traffic penalties, the Guardian has learned.

The figure puts America at the top of a list of foreign embassies and missions which have been refusing to pay the daily £8 charge. More than £10m in outstanding penalty charge notices (PCNs) is owed by 20 embassies, according to figures released by Transport for London (TfL) under the Freedom of Information Act.

The US embassy has been on a collision course with the mayor since July 2005 when, following state department orders, it declared the congestion charge an illegal tax under the Vienna convention and instructed diplomatic, administrative and technical staff to stop paying it. That led Livingstone to label the US ambassador, Robert Tuttle, a "chiselling little crook" in a TV interview in 2006. Although the mayor threatened to retaliate by clamping and removing cars belonging to diplomats, a crackdown has yet to happen.

A US embassy spokesman said it was wrong to consider the issue a personal row between Livingstone and Tuttle. "It is unfortunate that the mayor is taking a policy instruction from the department of state and inappropriately personalising it." Livingstone, whose comments saw him reported to a standards watchdog, was silent on the matter yesterday. But a spokesman for the mayor said: "The UK government has ruled that the congestion charge is a charge for a service - namely reduced congestion and traffic - and not a tax. This means that diplomats are not exempt from payment.

"The worst culprit by far is the American embassy, which has taken a deliberate decision to ignore UK law and refuse to pay the charge. As a result they now owe over £2m, which would have been invested in improving the transport network." He added that if the US expected diplomats in Washington to respect US law, the UK had the right to expect American diplomats in London to observe British law.

The biggest debtor after the US is the Japanese embassy, which owes more than £1m, followed by Nigeria's high commission (£982,350) and the embassies of Russia (£912,360) and Germany (£828,170). A spokesman for the Japanese embassy said its government had carefully studied the legal nature of the charge before ceasing payment on August 1 2006. "As a result, the government of Japan reached an understanding that the charge corresponds to neither 'dues and taxes ... such as represent payment for specific services rendered' nor 'charges levied for specific services rendered', as stipulated in the relevant international conventions," he said. "Therefore the embassy, its diplomatic agents and their family members should be exempt from the charge."

Germany began following a similar line last year, and other European embassies have met to discuss the congestion charge but have been unable to agree a common EU stance. Other EU states to feature in the top 20 embassies with outstanding PCNs are France (£263,200), Poland (£249,000), Romania (£203,740), Spain (£184,580) and Greece (£181,200).

The mayor's position is supported by the Foreign and Commonwealth Office, which points out that UK diplomats in Norway and Singapore are not exempt from congestion charges there. Other figures released by TfL show that the embassy of Saudi Arabia is the worst diplomatic offender when it comes to flouting parking laws. It owes £6,950, followed by Nigeria (£3,850), Guinea (£2,950), the United Arab Emirates (£2,700), Egypt (£2,300), Qatar (£2,250), and Brunei (£2,000).