Prime Minister David Cameron was described "unsure, inexperienced, oblique, and largely uncommitted" on foreign policy while London Mayor Boris Johnson was dubbed as the "Tory clown prince" in private mails shared with US presidential hopeful Hillary Clinton by one of her close confidants.

The observations were made by Sidney Blumenthal, an adviser to former president Bill Clinton when he was in the White House and a confidant of Hillary, in 2009 just before Cameron became prime minister, The Times reported.

Blumenthal, known as "Sid Vicious" in Washington's political circles, wrote to the former secretary of state, describing Cameron: "His political imperatives have pressured him to lean right. 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."

The emails were sent in a private system used by Hillary, who is running for US president in 2016, when she was the secretary of state.

Blumenthal added that a Tory government led by Cameron would be "more aristocratic and even narrowly Etonian ... sharply contrasting especially with the striving and classless perspective of the grocer's daughter, Margaret Thatcher".

The mails have surfaced just when the Obama administration is accused of interfering in Britain's internal affairs following Washington's suggestion to the UK to stay within the EU in order to gain greater global political influence.

Responding to the disclosures, which are likely to have an impact on the UK-US ties, Downing Street 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."