A proposed congestion charge for motorists to drive their cars into the centre of London has sparked a diplomatic row with the United States.

US embassy staff believe they should be exempt because the charge amounts to a tax.

The £5 charge is due to be implemented in February and will cover all but some professions, such as emergency services.

It is the idea of London Mayor, Ken Livingstone, and his office insists embassy staff will have to pay because it is a charge, like a parking ticket, not a tax.

We are aware the scheme will have a financial impact on diplomatic missions in London, especially those from developing countries

Foreign Office

Officials at the US embassy say if the mayor does not back down, British diplomats in Washington could find themselves paying a similar charge.

A US embassy spokesman said: "The US government is opposed to the application of the charge to US embassy staff and other diplomats because it is in violation of the 1961 Vienna Convention, which bars the taxing of foreign diplomatic staff."

He added: "We have made formal complaints to the mayor's office but he has declined to meet with us and we have brought the illegal nature of this tax to the attention of the British Government but it is unable to do anything about it.

"We are not opposed to what the mayor is attempting to achieve. This is a matter of principle."

The mayor defended the plan in court

He said there were 16 employees who drive to work, out of the 750 at the American Embassy in Grosvenor Square.

But a spokeswoman for the Mayor's office denied it was a tax and said: "They are not exempt and will have to pay the charge when it is brought in.

"People will only be exempt for social or financial reasons."

A Foreign Office spokesman agreed that the congestion charge did not amount to a tax.

Capital costs

He said they had looked at the matter carefully but in their view it did not warrant exemption for diplomats.

The spokesman added: "However, we are aware that the scheme will have a financial impact on diplomatic missions in London, especially those from the many developing countries already facing real difficulties in meeting the cost of maintaining a diplomatic presence in the capital.

US diplomats say the charge is a tax

"Ministers have already made representations to the Mayor's office on this point."

Last month in New York, intervention from the Secretary of State Colin Powell diffused a similar row when diplomats almost had their cars towed for parking offences.

Mr Livingstone won a High Court battle with Westminster Council to press ahead with the controversial charge in London.

The council had claimed the effect would be to push traffic out to surrounding areas, increasing air pollution and breaching residents' human rights.