London mayor Ken Livingstone has been reported to a local government standards watchdog after likening the US ambassador in London to a "chiselling little crook".
Mr Livingstone made the off-the-cuff comment while giving his opinion of the US embassy's decision not to make its diplomats pay the congestion charge when driving into the centre of the capital.
Directing his comments at the relatively new US Ambassador Robert Tuttle, Mr Livingstone said: "Since this new ambassador took over in July they have not paid.
"When British troops are putting their lives on the line for American foreign policy it would be quite nice if they paid the congestion charge. We will find a way of getting them into court either here or in America. We are not going to have them skive out of their responsibilities."
Later, he was even more forthright, telling ITV's London Today: "It would actually be quite nice if the American ambassador in Britain could pay the charge that everybody else is paying and not actually try and skive out of it like some chiselling little crook."
The civil liberties group Liberty and Law reported the mayor to the Standards Board for England, the body responsible for standards in local democracy. Liberty and Law believes Mr Livingstone breached the board's code of conduct on three counts.
Director Gerald Hartup said Mr Livingstone was in breach of the code's requirement that members must treat others with respect,
Secondly, he is said to have breached the requirement not to do anything which compromises the impartiality of those who work for the authority. Finally, he is said to have breached the requirement that he should not conduct himself in a manner which could "bring his office or authority into disrepute".
The Embassy says the £8-a-day congestion charge is a local tax and therefore, under the Vienna Convention, does not apply to foreign diplomats.