Sir Nigel has been the UK ambassador in Washington since last year
The British ambassador to Washington has described US presidential hopeful Barack Obama as "highly intelligent" but at times "aloof", reports say.
Sir Nigel Sheinwald also said Mr Obama was untested and if elected would "have less of a track record than any recent president", the Daily Telegraph said.
The comments came in a letter sent to UK PM Gordon Brown in July, it added.
Sarah Palin has meanwhile apologised after mistakenly saying she had met Sir Nigel in July, the Daily Mail reported.
The Republican vice-presidential nominee's campaign said a meeting between the two occurred at the US National Governors Association in Philadelphia, but an embassy spokesman said the ambassador had withdrawn at the last minute.
Ms Palin has faced strong criticism that she lacks experience in foreign policy. She only got a passport last year and had not met a foreign head of state until last week.
The UK embassy in Washington declined to comment on the reports of Sir Nigel's comments about Mr Obama.
In the seven-page letter reportedly sent on the eve of Mr Obama's visit to London in July, marked as containing "sensitive judgements", Sir Nigel is said to paint a mixed picture of the Democratic Party's presidential candidate.
The ambassador describes Mr Obama's speeches as "elegant" and "mesmerising" and states that he is "highly intelligent" and has "star quality", according to a copy of the document reportedly seen by the Telegraph.
But Sir Nigel is also said to warn that "there is little Obama track record to refer back to".
"Obama's politics and policies are still evolving," he is quoted as saying. "His Illinois and US Senate careers give us only a few clues as to his likely priorities in office.
"In the Senate... his voting record was decidedly liberal. But the main impression is of someone who was finding his feet, and then got diverted by his presidential ambitions."
Sir Nigel is also said to identify several weaknesses which Mr Obama's Republican rival, John McCain, is likely to exploit in the last month of campaigning.
Gordon Brown has been accused of taking sides in the US election
Mr Obama "can seem to sit on the fence, assiduously balancing pros and cons", he reportedly says, and "does betray a highly-educated and upper middle class mindset".
Charges of elitism "are not entirely unfair" and he is "maybe aloof, insensitive" at times, Sir Nigel is said to add.
The paper also quotes Sir Nigel as telling the prime minister that Mr Obama's stance towards Iran might conflict with UN Security Council demands that it stop enriching uranium before negotiations with the West resume.
However, the ambassador does say the Democratic candidate's policy on Iraq would "be consistent" with that of the UK government.
Last month, Mr Brown was accused by the opposition Conservative Party of taking sides in the US election after he praised the Democrats in a magazine article and cited one of Mr Obama's proposals to help families. Mr McCain was not mentioned.