Britain has dismissed US President-elect Donald Trump's unprecedented expression of support for Brexit campaigner Nigel Farage to be made British ambassador to Washington, saying pointedly that there is no vacancy for the job.

Mr Trump, who after his election victory met Mr Farage before any EU leaders, said on Twitter that "many people" would like to see the former metals trader turned politician as Britain's ambassador.

"Many people would like to see @Nigel_Farage represent Great Britain as their Ambassador to the United States. He would do a great job!" Mr Trump said on Twitter.

Analysis: 'A breach of etiquette' By Europe correspondent James Glenday At best Donald Trump's tweet could be described as unconventional. At worst some diplomats think the President-elect has set out to embarrass or make life awkward for the British Prime Minister. Publicly calling for a new ambassador is considered a breach of etiquette. Suggesting Nigel Farage — a controversial Brexiteer who has dedicated a large proportion of his life to fighting the current party of Government — should take the high-powered position tramples on most diplomatic protocols. Mr Trump must have known what the response would be and the feathers it would ruffle in Downing Street. Yet, the Government's response was very polite — too polite for some critics. Senior figures, including Boris Johnson and Theresa May, publicly criticised the President-elect in the lead up to the election campaign. Now they are desperately trying to reaffirm the "special relationship" the UK has long had with the US. Few expected Donald Trump to follow the established conventions of western politics and foreign policy. But even the "flattered" Mr Farage described this as a "bolt from the blue".

Prime Minister Theresa May, who congratulated Mr Trump on his victory, was swift to reject such an undiplomatic proposal.

"There is no vacancy," a Downing Street spokesman said when asked about Trump's remark on Tuesday.

"We already have an excellent ambassador to the US."

It is highly unusual in the modern era for leaders to publicly suggest to foreign nations whom they would like to see as ambassador, though during strained relations they sometimes reject or expel envoys.

Mr Farage, who spent decades campaigning for Britain to leave the European Union and helped force former Prime Minister David Cameron call the June referendum that brought the Brexit vote, spoke at a Trump rally during the US campaign and visited the President-elect after his victory.

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As leader of the UK Independence Party (UKIP) and one of the key figures of the successful Brexit campaign, Mr Farage has repeatedly angered EU leaders by predicting the collapse of the EU, which he says is run by an out of touch elite of "idiots".

Mr Farage said Mr Trump's suggestion that he serve as ambassador had come "like a bolt from the blue" but Mr Trump understood loyalty in a way that those in the "cesspit" of career politics did not.

"I am in a good position with the President-elect's support to help," Mr Farage said in an article written for the Breitbart news website.

"The world has changed and it's time that Downing Street did too.

"I would do anything to help our national interest and to help cement ties with the incoming Anglophile administration."

Reuters