With his wife Samantha about to give birth to the couple’s third child any day, Cameron says he will take one week immediately and “probably” another week soon afterwards “when the baby isn’t sleeping and Samantha’s more tired”.

William Hague, the former Tory leader, who spoke last month of his own ambition to start a family, will stand in for Cameron during prime minister’s question time.

The Camerons already have two young children: a three-year-old son, Ivan, who suffers from cerebral palsy and epilepsy; and a daughter, Nancy, aged two. When the baby arrives he or she will make Cameron the third party leader in six years to become a new father, highlighting the youthful image of modern politics.

When Leo Blair was born in 2000, he was the first child to be born to a serving prime minister since the Liberal premier Lord John Russell in 1849. Blair belatedly took two weeks paternity leave, but Charles Kennedy, the Liberal Democrat leader, was pitched straight into an election campaign when his son, Donald, was born last year.

Cameron reveals his paternity leave plans in an interview for the March issue of Cosmopolitan magazine. In a clear attempt to appeal to young mothers, the Tory leader also explains how he helps Samantha with the childcare. “We both get up and she does one baby, I do the other,” he says.

In the interview, he admits that he once bought a fur hat, but is not sure if it is real fur; that he has no ambition to become a Cosmopolitan centrefold; that he has never examined himself for testicular cancer and that he would like the abortion limit reduced from 24 to 22 or 20 weeks; and Cameron also says he supports a new law to prevent stalking, mainly carried out by men against their ex-girlfriends. “It simply doesn’t work,” he says of the law. “Stalking should be a criminal offence. We need to look at the law and, while in opposition, push the government to get it right.”

Cameron steers clear of giving a straight answer on whether he took drugs while at Brasenose College, Oxford, in the 1980s. He says only that he enjoyed “the usual university experience”.

He also speaks of the effect the Aids scare of the 1980s had on university life. “When I went to university in 1985 there was the first big public ad campaign about Aids — Don’t Die of Ignorance. It scared the life out of us all, and it changed people’s attitudes. I think we need those sort of hard, gritty campaigns again on drugs, on sexually transmitted infections.”

The forthcoming addition to the Cameron family has already had a political impact: party conference pictures of the expectant father patting his wife’s bump helped steer him towards the leadership. So will the birth have any effect on the opinion polls as the Conservatives try to overtake Labour?

“When we win a big sporting event then a politician’s rating goes up, but a birth doesn’t really have an effect for them,” said Ben Page, of Ipsos Mori, the opinion pollsters. “A new baby is nice to have but it’s not a knock-out blow. I don’t think it will have an adverse effect but I doubt it will be beneficial.”

In fact, Tony Blair suffered a 2% fall in popularity following the birth of Leo. That should be a reminder to David Cameron that politicians who involve their families in political campaigning are playing with electoral fire. Cameron has already used the plight of his son to highlight failings in the NHS, but is well aware of the risks.

“There must be a way of being a good politician and a good parent,” he once said. “But if there is a way, very few politicians of the front rank have found it. Carol Thatcher is far from the only daughter or son to testify to how unhappy are the lives of the children of many leading politicians.”

Cameron’s Cosmopolitan interview comes as he prepares to launch his latest policy taskforce this week. The democracy review, to be headed by Kenneth Clarke, the former chancellor, will, among other issues examine whether Scottish, Welsh and Northern Irish MPs should be able to vote on English law at Westminster.

Even on paternity leave there will be no escape from politics for Cameron. As he tries to outflank Labour on the environment, will he and Samantha be using disposable nappies or making a bold statement with the washable version? A spokesman said: “The Camerons do not use reusable ones.”

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