The US Embassy has failed to pay £7.2 million worth of congestion charge fines in London.
Foreign Secretary William Hague said the British authorities are owed the sum as a result of more than 63,000 fines.
The figures were released as part of the annual statements of outstanding fines, penalties and taxes not paid by foreign embassies.
The US figure far outstrips the nearest rival - the Russian Federation, which owes £4.89 million from 42,310 fines. In a close third is Japan, which owes £4.85 million from 42,206 fines.
Diplomats also fail to pay parking fines in London, Mr Hague said, with net £344,747 unpaid in 2012 alone.