They never tire of telling us about the importance of marriage, so why don't British prime ministers ever wear a wedding ring?

David Cameron is proud of the fact that he does not wear a watch or a wedding ring. Some have put this down to the sartorial conservatism of the Tory male. Or a certain upper crust insouciance.

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But maybe he is just following in the footsteps of his predecessors at No 10. Gordon Brown is well known for selling off Britain's gold reserves - does that explain his lack of a wedding band?

Tony Blair may have felt the hand of history on his shoulder but he never felt the reassuring grip of a gold band on his ring finger. A habit he continues to this day.

In fact the last occupant of No 10 to regularly sport a wedding ring in public was Lady Thatcher.

Even John Major - cruelly mocked for his suburban conformity - did not wear one. Here he is demonstrating his bold defiance of convention to German Chancellor Helmut Kohl.

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They would not have got away with it in America, where maintaining the image of a perfect presidential marriage is paramount. When Barack Obama's wedding ring briefly disappeared from his finger it was front page news (it turned out that it had been sent for repair). Obama's vice-president Joe Biden does not wear a ring though. Much to the irritation of conservative commentators.

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The last married male occupant of 10 Downing Street to wear a wedding ring in public was Harold Wilson, who was PM from 1964 to 1970 and 1974 to 1976.

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Wilson's successor "Sunny Jim" Callaghan was married but ringless. Tory PM Ted Heath was a lifelong bachelor. Wilson's aristocratic opponent in the 1964 general election, the happily married Sir Alec Douglas-Home may have renounced his peerage - but he was never Lord of The Rings.

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Gentlemen did not wear jewellery in Douglas-Home's day. "Tie clips, cufflinks the size of saucers and all rings with stones are absolutely out. A signet ring on the little finger of the left hand is, however, perfectly all right," wrote the Duke of Bedford in his 1965 tome The Book of Snobs.

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Winston Churchill, typically, defied convention by wearing his signet ring, sporting his family's crest, on his ring finger. His Labour opponent in 1945, Clement Attlee, wore a pinkie ring, as the Americans call them.

But the convention for men wearing wedding rings on the fourth finger of their left hand did not really take off until World War Two, when serviceman fighting overseas wanted to wear a reminder of their wives and families back home.

As a relative latecomer to wedded bliss, we can perhaps forgive Labour leader Ed Miliband for wanting to tell the world about it. But if he is serious about joining the world leaders' club, maybe he should take a leaf out of German Chancellor Angela Merkel's book and sling the ring.