Politicians have sexual affairs because they want to break out of the "prison of self-control" which life in politics imposes on them.

This is according to British ex-premier Tony Blair said in his memoirs which became an instant bestseller this week.

Politicians had to be extremely controlled to get anywhere, he wrote, adding: "And your free-bird instincts want to spring you from that prison of self-control".

"Then there is the moment of encounter, so exciting, so naughty, so lacking in self-control.

"Suddenly you are transported out of your world of intrigue and issues and endless machinations and the serious piled on the serious, and just put on a remote desert island of pleasure, out of it all, released, carefree," he wrote.

However, Blair who has been married to lawyer Cherie Booth for 30 years with four children, insisted such inclinations were "not an excuse," PA reported.

He also added the old saying that power was an aphrodisiac had to be true: "Let's face it, most politicians are definitely on the debit side of the good-looks ledger," he said.