Sir Nigel Sheinwald, Britain’s Ambassador in Washington, has made a habit of ruffling feathers. His recent performance has, however, been exceptional, causing varying levels of irritation to the campaigns of both US presidential nominees.

The British Government was left squirming with embarrassment yesterday after the leak of a confidential letter to Downing Street in which Sir Nigel mixed praise for Barack Obama with a description of his “liberal voting record” and a suggestion that charges of elitism “are not entirely unfair”.

But the ambassador has also sent ripples of annoyance through Republican ranks. The Times has learnt that last summer he cancelled a meeting with Sarah Palin - before she was picked as vice-presidential nominee - on the ground that talking to the Governor of Alaska would be a “complete waste of my time”.

He has since, unsuccessfully, made efforts to arrange a new appointment. But the subject resurfaced at the worst possible time on Thursday before Mrs Palin’s TV debate with her Democratic rival when her aides, seeking to bolster a thin résumé, mistakenly listed Sir Nigel as one of the world figures she had met. The British Embassy swiftly forced the McCain campaign to retract the remark by issuing a reminder that the meeting, at a US governors’ conference in July, had not gone ahead.

An embassy spokesman yesterday disputed the account - from two separate sources - of Sir Nigel’s reasons for not seeing Mrs Palin, saying: “The bottom line is that he did not go to the meeting because he had other things to do.” One Republican strategist suggested that “we will be able to forgive the ambassador this time” - partly because of the ammunition he provided with his letter on Mr Obama.

Susan Rice, Mr Obama’s foreign policy adviser, insisted there was “nothing insulting in the letter if you read it in its entire context”. Asked if she thought that Sir Nigel’s assessment was fair, Dr Rice replied: “No comment.” She added: “This is an awkward moment for the embassy and for London. I don’t see any reason to make it more awkward.”

Sir Nigel’s often florid telegrams to London have been a source of amusement and sometimes alarm within Whitehall for much of the past year since being appointed to his post by Tony Blair, for whom he worked as foreign policy adviser. Although he has worked hard to maintain a high level of contact with both campaigns, some officials have voiced concerns privately that his tendency to opine - combined with some rocky staff relationships in the embassy - were becoming a diplomatic danger zone.

Sir Nigel had banned officials from discussing the campaign with journalists and severed virtually all contact with the press to guard against just this kind of damaging disclosure.

His letter to Gordon Brown was written last summer before Mr Obama’s tour of European capitals. He extols the Democrat for having “star quality” and being “highly intelligent”, but adds: “If elected, Obama would have less of a track record than any recent president. Carter would be the nearest, but even he had four years as a Governor.” Sir Nigel also makes explicit at least one area of potential policy difference. “If Obama wins, we will need to consider with him the articulation between (a) his desire for ‘unconditional’ dialogue with Iran and (b) our and the [UN Security Council]’s requirement of prior suspension of enrichment before the nuclear negotiations can begin,” he wrote. The Embassy has repeatedly denied that Britain is worried about Mr Obama’s policy of direct talks with Iran.

Sir Nigel expressed pessimism on the Middle East peace process, saying: “It is unlikely to be a top priority for Obama.” Mr Blair, the regional peace envoy, has been told by the Democrat that progress is unlikely within the initial two years of his administration.

On Her Majesty’s Service

Born: June 26, 1953

Education: Harrow, Oxford

1976 Joined Diplomatic Service

1978 Second Secretary, Moscow

1979 Rhodesia/Zimbabwe Department, FCO

1981 European and Soviet Department, FCO

1983 First Secretary, Washington

1987 Deputy Head, Policy Planning

1989 European Community Department (Internal), FCO

1993 Counsellor and Head of Chancery, Representative to EU

1995 Head of News, FCO

1998 Director of the EU, FCO

2000 Ambassador to EU

2003 Adviser to the Prime Minister

2007 Ambassador to Washington

Source: Times archives

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