Foreign embassies in London have built up £23m in penalties for unpaid congestion charge fees since the levy was introduced in February 2003.
The US Embassy has the highest outstanding payment of £2.7m, followed by Russia at £1.8m and Japan which owes £1.7m to Transport for London (TfL).
The £8-a-day charge is mandatory for motorists entering central London.
Mayor Boris Johnson said he was "disappointed" at the refusal of some embassies to pay the congestion charge.
TfL said diplomats were not exempt as the charge is a "service" but some embassies have refused to pay, calling it a "local tax".
Under the Vienna Convention on Diplomatic Relations, local taxes do not apply to foreign diplomats, giving diplomatic immunity from enforcement.
A total of 123 embassies and high commissions owe congestion charges, in figures revealed by Labour members of the London Assembly.
Between 2003 and October 2008 the US embassy failed to pay the charge on 26,165 occasions, the Russian embassy ignored the levy 17,292 times and Japan has 16,657 unpaid charges.
Mr Johnson said: "I am disappointed that some embassies continue to refuse to pay the charge even though TfL and the government's position on this matter is clear and consistent.
"However, it is pleasing to note 77% of embassies pay the charge on a regular basis as they are required to do."
A spokesman for the US Embassy said that while the embassy pays for any traffic violations, fines or parking tickets, it considers the congestion charge to be a direct tax and therefore does not pay.
"Former Mayor Ken Livingstone himself admitted in 2003 that the congestion charge qualified as a direct tax," the spokesman added.
A TfL spokesperson said: "TfL and the UK government are agreed that the congestion charge is a charge for a service and not a tax, which means that diplomats are not exempt from payment."
Following the expansion of the congestion charge zone to include Kensington and Chelsea last year, a further 60 foreign missions came within the charging area.