A key confidant of US presidential candidate Hillary Clinton told her David Cameron was "unsure, inexperienced, oblique and largely uncommitted" on foreign policy, it has been revealed.
The assessment came from Sidney Blumenthal in 2009, shortly before Mr Cameron became Prime Minister, and was contained in an email from a private system used by Mrs Clinton when she was US secretary of state.
Other messages describe London mayor Boris Johnson as "the Tory clown prince" and describe efforts by Gordon Brown and Ed Balls to prevent David Miliband from inheriting the Labour leadership as being "like the Tudors", The Times said.
The paper quoted Mr Blumenthal as telling Mrs Clinton - who is currently seeking the Democrat nomination to succeed Barack Obama in 2016 - that a Cameron government would be "more aristocratic and even narrowly Etonian ... sharply contrasting especially with the striving and classless perspective of the grocer's daughter, Margaret Thatcher".
Mr Blumenthal, who was an aide to Bill Clinton during his time in the White House, said Mr Cameron's foreign policy was largely driven by his "need to keep his party behind him" on issues such as Europe.
"His political imperatives have pressured him to lean right," he wrote. "His future problem is that he does not want to be the leader who takes Britain out of Europe but he is putting himself in a position where he may not be able to prevent his party doing so."
In another email, Mr Blumenthal warned that the UK-US alliance was being destroyed by "the Obama administration's denigration of the UK" and the president's indecision over Afghanistan.
The messages were among tens of thousands of pages of emails handed over to the US State Department by Mrs Clinton earlier this year, following controversy over her use of a private email system for official business.
A Downing Street spokeswoman said: "The special relationship between the UK and the United States is as strong as ever and the president recently acknowledged the importance of the alliance to the US."