A new documentary examining the political influences on Conservative leader David Cameron has revealed that he was persuaded to drop his support for Section 28 by his wife Samantha.

Nick Boles, the Tory prospective parliamentary candidate for Grantham and Stafford and a close friend of the couple, told the Radio 4 programme that Mrs Cameron was instrumental in turning her husband into a moderniser.

Mr Cameron, who became an MP in 2001, married Samantha in 1996.

Mr Boles, who is gay, said that with regard to Section 28, which prohibited the “promotion” of homosexuality in schools:

“I think David didn’t understand for a long time why this was as utterly obnoxious as it was, and Samantha was much more clear-eyed about seeing that it was an attempt to stigmatise a particular group totally illegitimately.”

Mr Boles, the former director of the Policy Exchange think tank, was a strongly-tipped candidate for the Tory nomination for Mayor of London, but withdrew in July 2007 when he was diagnosed with Hodgkins Lymphoma, a type of cancer.

The party’s nomination was won by fellow Cameron ally Boris Johnson yesterday.

An influential member of the “Notting Hill set,” Mr Boles was a parliamentary candidate in the 2005 election. He came within 500 votes of winning Hove.