'Many here feel this was an ignoble attitude from the ambassador of the wealthiest country on earth, and one that has set an unfortunate tone, and a poor example for other embassies to follow,' he wrote.

However, two weeks after new Ambassador Louis Susman took charge, Washington has refused to change its policy.

A spokesperson for the State Department said: 'There has been no change in policy regarding the congestion tax. This is a long standing US policy and is not changed by the change in ambassador. We believe the charge to be a tax that is prohibited by various treaties.'

As of June 3, the US embassy owed Transport for London - the local government body responsible for most aspects of transport - 3,478,200 pounds in unpaid fines, part of a whopping 28 million pounds owed by all diplomatic missions in London.