David Cameron privately described opponents of gay marriage as 'Neanderthals', according to a new biography on the Prime Minister.
The revelation came in a biography, written by Sir Anthony Seldon and Peter Snowdon, which was compiled from over 300 interviews with senior figures at Number 10.
Sir Anthony said that the issue of gay marriage was very divisive in the party, with some calling it a "self-inflicted wound."
He wrote that Mr. Cameron had considered including redefining marriage in his 2010 Conservative manifesto.
Close advisers cautioned that the policy could be a distraction.
In response to his advisers repeatedly warning him against gay marriage, the Prime Minister is quoted as saying: "Unless you are making some Neanderthal judgement on gays, those who are gay should have the same rights as those who are not."
Sir Anthony also said that Mr. Cameron had empathy with same-sex couples because he had friends at university who were gay.
The Steerpike blog on The Spectator website said ahead of the book's release that concerns were beginning to grow about the accuracy of the biography.
The blog also said that some people who had been written about were not given the opportunity to check draft chapters.
More than half of the Conservative Members of Parliament voted against redefining marriage in February 2013, but the legislation was passed and same-sex couples were able to marry from last year.
During the passage of the legislation, former Deputy Prime Minister Nick Clegg referred to Peers voting against gay marriage as "dinosaurs". His office was also forced to retract a speech, issued to the media, which called opponents of gay marriage "bigots."
Clegg said he never intended to make the remark and that it was a drafting error by his officials.
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