The U.S. embassy in London is pointing to a 63-year-old diplomatic convention to avoid paying a $18.7 million congestion charge bill

The U.K.—U.S. “special relationship” has had a few hiccups over the years—the Suez Crisis, disagreements over the Vietnam War, and now 20 years’ worth of unpaid traffic charges.

The U.S. is in a feud with the British capital’s transport body, Transport for London (TfL), over a £15 ($19) daily congestion charge for drivers in the center of London.

Diplomats driving through the city and other U.S. embassy-linked cars have racked up £14.7 million ($18.7 million) in unpaid congestion charges since 2003, according to TfL data.

The resultant hole in TfL’s finances has set off a debate over whether the U.S. and other embassies are even liable to pay the fees, which are designed to improve air quality and reduce journey times in the busy city.

While embassies are entitled to exemptions and reductions on certain taxes, including income tax, TfL says the congestion charge doesn’t fall under this umbrella because it is a service.

And the transport body is now threatening to take the embassy to an international court to make its case.

“The majority of embassies in London do pay the charge, but there remains a stubborn minority who refuse to do so, despite our representations through diplomatic channels,” TfL wrote.

“We will continue to pursue all unpaid Congestion Charge fees and related penalty charge notices and are pushing for the matter to be taken up at the International Court of Justice.

In total, TfL says foreign embassies owe a total of £143.5 million ($183 million) in congestion charges. The Japanese embassy trails the U.S., with the Indian embassy having the third biggest disputed debt.

The U.S. embassy is standing firm on its unpaid bills, pointing to a 63-year-old convention to claim diplomatic immunity.

“In accordance with international law as reflected in the 1961 Vienna Convention on Diplomatic Relations, our position is that the Congestion Charge is a tax from which diplomatic missions are exempt,” a US embassy spokesperson told CNN.

“Our long-standing position is shared by many other diplomatic missions in London.”

A representative for the embassy didn’t immediately respond to Fortune’s request for comment.

The U.S. decided to stop paying the congestion charge for its diplomatic vehicles in the city in 2005, at the time citing the Vienna Convention.

The London government is siding with TfL, arguing that the U.S. and other embassies are required to pay the charge under the Congestion Charging Scheme Order, with a representative of the U.K. Foreign Office telling CNN that they believed the U.S. wasn’t exempt on the grounds of diplomatic immunity.

Fears of any kind of escalation are unjustified, however. TfL has been complaining about unpaid congestion charges for as long as they've been unpaid (more or less from day 1). It’s also hardly the most egregious example of diplomats ignoring traffic rules.

At the turn of the millennium, Kuwaiti officials at the UN averaged nearly 250 New York City parking violations each, over a five year period, but relations with the Gulf nation are still cordial. The U.K. averaged zero, incidentally, but who’s counting?

This story was originally featured on Fortune.com