US presidential candidate Hillary Clinton’s contempt for David Cameron before he became prime minister has been revealed in the release of her private emails.

The emails show that as US secretary of state, Clinton was sent Guardian articles by a key confidant, Sidney Blumenthal, to highlight Cameron’s inexperience.

One of the articles shared with Clinton reported how in October 2009 Cameron had incensed the then leaders of France, Germany and Spain over an attempt to scupper the Lisbon treaty.

Clinton described the Guardian article as “so revealing and wacky”. She also speculated that it could boost a then flagging, and ultimately futile bid, by Tony Blair for the EU presidency.

In his advice to Clinton, Blumenthal was scathing in his assessment of Cameron before he became prime minister. In another email from earlier in October 2009 he wrote: “On foreign policy, Cameron is unsure, inexperienced, oblique, and largely uncommitted. So far his foreign policy is little more than projection of his domestic politics.”

He also highlighted another Guardian article reporting how a French minister accused the Conservatives of “castrating” Britain’s position within the EU by adopting an “autistic” approach to Europe.

In the subject line of the email Blumenthal wrote: “Decline and fall of the British Empire, 2009 edition.”

A few weeks later Blumenthal looked ahead to the prospect of a Cameron premiership. “A Cameron government would be more aristocratic and even narrowly Etonian than any Conservative government in recent history, sharply contrasting especially with the striving and classless perspective of the grocer’s daughter, Margaret Thatcher,” he wrote.

He also warned that if elected “Cameron would be superficially friendly [towards the Obama administration] and privately scornful.”

Earlier in the year Clinton wrote that she was “very sorry to read” confirmation that Labour was on course to suffer a large defeat in the European elections, a year before its general election defeat in 2010.

The emails also revealed the extent to which Clinton was prepared to lobby for Blair to become president of the EU.

Blumenthal was worried that Blair would not be chosen for a post that was eventually won by Herman Van Rompuy. He wrote: “If Blair does not become EU president the position will likely be filled by a third rank nonentity in the Brussels bureaucratic mode incapable of realising the possibilities in the creation of the office, continuing the feebleness of Europe as a political idea and reality.”

He also revealed that Blair’s former chief of staff Jonathan Powell wanted Clinton to publicly endorse a “strong candidate” for the role.

In an email to Clinton, Blumenthal wrote: “I spoke with Jonathan Powell this week and he tells me the following: ‘The argument is slipping away from us a bit at the moment so anything Hillary can do would be gratefully appreciated in speaking to other European leaders or what she says in public about about the need for Europe to have a major figure at its head so that other powers can relate to it.’”

Blumenthal’s assessment of Cameron was remarkably similar to the one made by Mervyn King, then governor of the Bank of England, as revealed in the leaks of diplomatic cables. He said Cameron and George Osborne operated too much within a narrow circle and “had a tendency to think about issues only in terms of politics, and how they might affect Tory electability”.

The messages to Clinton were among tens of thousands of pages of emails handed over to the US State Department by 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.”