Sir Nigel Sheinwald, Britain's Ambassador in Washington, was last night thrust into political controversy after the leak to a newspaper of a seven-page candid assessment he delivered on Barack Obama.

His letter to Downing Street, written earlier this summer ahead of the Democratic nominee's trip to Europe, contains a level of criticism that will be deeply embarrassing to the Government.

He describes Mr Obama's "decidedly liberal" voting record in the Senate where "the main impression is of someone who was finding his feet, and then got diverted by his presidential ambitions".

Some of his comments, suggesting there is "little track record to refer back to" and that charges of elitism or being aloof "are not entirely unfair" may be seized upon by John McCain's campaign which has consistently sought to portray Mr Obama as inexperienced and out-of-touch with working class voters.

The letter to Number 10 predicts there is a potential clash with a future Obama Administration over Iran, even though British Embassy officials have consistently gone on record denying such claims.

"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 [United Nations Security Council]'s requirement of prior suspension of enrichment before the nuclear negotiations proper can begin," wrote Sir Nigel.

For several months, British diplomats have been warned by Sir Nigel to be careful about making any comments that might impact the US presidential race. The Ambassador himself became "highly agitated" according to one source when an article praising Mr Obama appeared under Gordon Brown's name last month.

Sir Nigel's letter is complimentary about the Democrat in places. Mr Obama's speeches are "elegant" and "mesmerising", he is "highly intelligent" and has "star quality", he said. But he also warns that "policies are still evolving" and that if elected he will "have less of a track record than any recent president".

While approving of his "progressive position on climate change" and "balanced approach to the big security issues", he raises concerns over Mr Obama's protectionist leanings on trade.

A British Embassy spokesman said last night: "We are not going to comment on a leaked document. We are totally neutral."

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