Barack Obama is a "decidedly liberal" senator "who was finding his feet, and then got diverted by his presidential ambitions", according to a frank verdict delivered to Gordon Brown by the British ambassador to the United States.
Gordon Brown and Barack Obama during their meeting at Downing Street Photo: GETTY
Sir Nigel Sheinwald, ambassador in Washington since last year, delivered his unvarnished assessment of the White House front runner in a seven-page letter to the Prime Minister, obtained by The Daily Telegraph, just before the Democratic nominee's visit to Downing Street just over two months ago.
The candid letter, marked as containing "sensitive judgements" and requesting officials to "protect the contents carefully" gives a remarkable insight into how the Foreign Office views the political phenomenon who stunned Mr Brown's inner circle by defeating their favourite, Hillary Clinton, in the Democratic primaries.
Although the picture Sir Nigel paints is a highly complimentary one - Mr Obama's speeches are "elegant" and "mesmerising", he is "highly intelligent" and has "star quality" - he also judges that his "policies are still evolving" and that if elected he will "have less of a track record than any recent president".
The letter's contents suggest that Mr Brown could initially find it difficult to deal with a President Obama because he remains a largely unknown quantity who "resists pigeon-holing" and the leak is likely to complicate relations.
Last month, the prime minister was forced to backtrack after an article written in his name broke with convention by showering praise on the Democratic candidate at the expense of his Republican rival, Senator John McCain of Arizona.
Sir Nigel traces the ambition of Mr Obama, 47, to reach the White House right back to his 20s or before. "He has talked at least since the 1980s about a shot at the Presidency."
He also identifies several political vulnerabilities that Sen McCain will seek to exploit in the last month of his campaign against the Illinois senator. The leaked letter will provide him with welcome ammunition.
Mr Obama "can seem to sit on the fence, assiduously balancing pros and cons", Sir Nigel wrote, 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.
"He can talk too dispassionately for a national campaign about issues which touch people personally, eg his notorious San Francisco comments [in April] about small-town Pennsylvanians 'clinging' to guns and religion."
Mr Obama's Democratic primary victory over the former First Lady showed that "he is tough and competitive. This is of course the Chicago school.
You don't beat Clinton without being resilient" but "his energy levels do dip and he can be uninspiring e.g. in debates".
Curiously, there is no mention of his wife Michelle - a central figure in his rise and his closest adviser - and little examination of his time in Chicago, where he had radical associations, or his background in Hawaii, essential to understanding why "Obama is cool", as the letter puts it.
Sir Nigel detects a potential clash between Downing Street and an Obama administration over Iran.
"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."
But Sir Nigel - who described the Iraq war as "the Iraq expedition" and "Bush's Iraq adventure" - briefed that Mr Obama's Iraq policy gelled with Britain's.
"Whatever the detail, our own proposed transition in south-east Iraq would be consistent with Obama's likely approach. Obama's ideas on a more expansive regional framework for Iraq would also fit well with our thinking."
He wrote approvingly of Mr Obama's "mainstream team of youthful economic advisers, with strong credentials [who] approach policy with refreshingly few prescriptions", his "progressive position on climate change" and his 'pragmatic realism" and "balanced approach to the big security issues".
Sir Nigel concludes that searching for a deal between Israel and the Palestinians is "unlikely to be a top priority for Obama" and he expresses concern about his protectionist trade policy, while noting that he has "repositioned himself somewhat towards free trade".
British officials said that since it became clear that Mr Obama would overcome Mrs Clinton, Sir Nigel had worked hard to dampen down what he viewed as "Obamamania" within Downing Street that had become so strong that he feared it might alienate the McCain campaign.
Sir Nigel's letter, though initially drafted by his political staff, is an intensely personal assessment of Mr Obama and is based largely on the ambassador's owns observations from the campaign trail.
He has travelled to rallies as far afield as New Hampshire and South Carolina - where he had a personal meeting with Mr McCain - and to the party conventions in Denver and St Paul, Minnesota.
Although he has the lowest public profile of any recent British ambassador in Washington, Sir Nigel has won respect for his range of contacts within the Bush administration and the campaigns.
The letter quotes Tom Daschle, a former Senate Majority Leader and Obama confidant who is hotly tipped to become White House chief of staff should Mr Obama be elected, from a private meeting with Sir Nigel.
There is a strong indication Sir Nigel also consulted Senator Richard Lugar, a Republican who has worked with Mr Obama on legislation and travelled abroad with him.
"Obama's politics and policies are still evolving," Sir Nigel wrote.
"His Illinois and US Senate careers give us only a few clues as to his likely priorities in office.
"In the Senate he took a low profile in 2005-6, but was a diligent member of the Foreign Relations Committee, respectful and friendly to the veteran Republican Senator Lugar, with whom he travelled to London in 2005.
"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 later reiterates the point: "Although he has been a member of the Senate Foreign Relations Committee for four years, and a regular attender of meetings in his first two, there is little Obama track record to refer back to."
He highlights luck as a key factor in Mr Obama's rise. "He was certainly lucky in having Democratic and Republican opponents for the US Senate in 2004 who were tarnished. He was lucky that Hillary Clinton had such a bad organisation in the primary campaign, and took so long to respond to Obama's threat."
A spokesman for the British Embassy in Washington declined to comment.
By Toby Harnden in Washington