Ken Livingstone has been reported to a local government standards watchdog after likening the US ambassador in London to a "chiselling little crook".

The Mayor of London 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.

Mr Livingstone’s latest outburst came in a row over whether diplomats at the US Embassy should pay the £8-a-day congestion charge.

Since last summer, when the charge increased from £5 to £8, the Embassy has maintained that it is a local tax and therefore, under the Vienna Convention, does not apply to foreign diplomats. It now owes more than £150,000.

Mr Livingtone’s ire was directed at the relatively new US Ambassador Robert Tuttle. The Mayor 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. This new ambassador is a car salesman and an ally of President Bush. This is clearly a political decision."

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."

A spokesman at the US Embassy said the decision had nothing to do with the arrival of Mr Tuttle and added that British diplomats are not taxed in the US.

Liberty and Law, a civil liberties pressure group, today reported the Mayor to the Standards Board for England, the body responsible for standards in local democracy including the GLA, claiming that Mr Livingstone had breached the Board’s code of conduct on three counts.

Gerald Hartup, the group's director, said that the Mayor 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."

Mr Livingstone was previously reported to the Standards Board over comments likening a Jewish reporter to a Nazi concentration camp guard. An Adjudication Panel subsequently suspended him for a month, but the penalty has not yet been enforced while Mr Livingstone mounts a legal appeal.

The Standards Board for England and the Adjudication Panel incurred costs of £50,000 in the investigation into Mr Livingstone’s comments to Oliver Finegold of the Evening Standard.

Murad Qureshi, a Labour member of the London Assembly, said the latest referral would also be a waste of money.

He said: "This is yet another waste of time and money and clearly politically motivated. It seems that the US government can flout British law but the mayor can’t have the right to condemn the US embassy for doing so. The US should start respecting our laws."

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