British Prime Minister David Cameron has privately described those who oppose same-sex marriage as "Neanderthals," a new biography written by Sir Anthony Seldon and Peter Snowdon said.

The book, entitled "Cameron at 10: The Inside Story 2010-2015," comprised of 300 interviews with senior figures at 10 Downing Street, according to Charisma News.

Sir Anthony said the issue of gay marriage has split the Conservative Party, with some members even calling it a "self-inflicted wound."

Cameron is for gay marriage, and even considered redefining the traditional marriage in his 2010 Conservative manifesto. However, he was heavily advised against it, Sir Anthony said.

In response, Cameron reportedly told his advisers: "Unless you are making some Neanderthal judgment on gays, those who are gay should have the same rights as those who are not."

Sir Anthony explained that Cameron was sympathetic to the plights faced by homosexuals, especially since he had friends during his university days who were gay.

However, there were some who questioned the biography's accuracy. The Steerpike blog of The Spectator website said that people should "take some of these stories with a pinch of salt for now."

"Anthony Seldon and Peter Snowdon's biography of David Cameron has not even been released yet, but already it has managed to send ripples through Westminster," the blog said.

While it is true that Lord Anthony got Number 10 aides to read and double check the draft chapters just to verify its accuracy, the same courtesy was not extended to the other parties who have been included in the biography.

One such person is London Mayor Boris Johnson, who allegedly received a text from Cameron telling him to "shut up."

But Johnson claimed that he and his staff have never received that offensive text message from the Prime Minister.