London mayor Ken Livingstone has been reported to a standards watchdog after likening the US ambassador in London to a "chiselling little crook".
The mayor has already fallen foul of the Standards Board once
Mr Livingstone made the comments during an interview about US diplomats failing to pay the £8-a-day congestion charge.
Campaign group Liberty and Law said it had reported the mayor to the Standards Board for England for breach of its code on three counts.
He is already appealing against a four week suspension for another comment.
The mayor attacked US ambassador Robert Tuttle after vowing he is prepared to go to court to make embassy staff pay the charge on vehicles entering central London.
However, the embassy argued it is a local tax which, under the Vienna Convention, does not apply to foreign diplomats.
But Mr Livingstone said: "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."
Gerald Hartup, Liberty and Law director, said the mayor was in breach of the code's requirement members must treat others with respect.
Mr Livingstone is also accused of breaching the requirement not to do anything that compromises the impartiality of those who work for the authority.
He is also accused of acting against the requirement he should not conduct himself in a manner that could "bring his office or authority into disrepute."
Mr Livingstone was previously reported to the board over comments likening a Jewish reporter to a Nazi concentration camp guard.
He was suspended for a month by its adjudication panel but is currently appealing against the decision.
Last week he become embroiled in a new row after suggesting two Jewish businessmen involved in building a key facility for the 2012 Olympics "go back and see if they can do better under the ayatollahs".
On Tuesday Conservatives on the London Assembly said they agreed the US embassy, along with 55 others, should pay the charge, but criticised the mayor's "outburst".
"I am not surprised that people have taken offence to these off hand and irrational remarks," said assembly member Bob Neill.
"It seems that these gaffes are coming on a weekly basis now."
No-one from the mayor's office was available to comment