Ken Livingstone has defended his calls for the US ambassador to pay the £8-a-day congestion charge.
The mayor has already fallen foul of the Standards Board once
The London mayor told the BBC that the American embassy's decision not to pay the road toll was "unacceptable" as it would cost Londoners more.
Mr Livingstone has been reported to a standards watchdog for comments he made about the ambassador for not paying.
The embassy said the charge is a local tax which, under the Vienna Convention, does not apply to foreign diplomats.
Mr Livingstone told Andrew Marr on Sunday AM: "I have to say I do think it is completely and utterly unacceptable that the American ambassador turns up, having made his billions selling cars, and they stop [paying].
Breach of code
"Particularly at a time when we are the only serious ally that America's got and our young people are putting their lives on the line for George Bush's foreign policy every day.
"I think it stinks that he's weaselling his way out of paying his fair share to London because it makes Londoners have to pay more because he's not paying his way."
Campaign group Liberty and Law said it had reported the London mayor to the Standards Board for England for breaching its code over comments he made about the US ambassador Robert Tuttle.
Mr Livingstone is already appealing against a four week suspension for another comment.
Gerald Hartup, Liberty and Law director, said the mayor was in breach of the code's requirement that members must treat others with respect.
The London Assembly Conservatives have backed the mayor's calls for the US staff to pay the charge, but criticised his outburst.