Candles and curry at Blair family birthday party

While studying at St John's College, Mr Blair was a member of the Archery Club dining society who posed for this group picture in 1975.

Although the photo has been published before, Mr Blair's hand gesture - which requires no explanation - was digitally removed from it by the press agency which supplied it, out of respect for the Prime Minister.

AFTER: Blair's hand mysteriously disappears

One of his contemporaries revealed that the young Blair was lucky to escape a second brush with the law after "flashing" from a college window as a student prank.

Peregrine Sharrock gave a vivid account of how, in his final year reading law at St John's, Blair dropped his tennis shorts in the hope of attracting the attention of female staff at a solicitor's office across the road.

Fortunately for him, the women either failed to notice his full-frontal display or turned a blind eye to it, as no complaint was made to the college and the incident was forgotten - until now.

Mr Sharrock recalls Mr Blair coming into his third-floor room in Dolphin Quad at St John's at around 2pm.

"He'd just had lunch and he and his tennis partner popped into my room for a coffee before going off to play tennis," said Mr Sharrock.

"He pulled down his shorts and appeared to show off his private parts to women in the office over the road. It was a solicitor's office which was only about 35 feet away, and was mainly full of secretaries working on old-fashioned typewriters.

"He said something like, 'Here you go, girls' and dropped them for about ten seconds. I don't know if anybody saw him but we never got a complaint.

"I was quite surprised. It was quite out of character for him. I have no idea why he did it. He wasn't drunk. I think he was just being silly.

"Tony could have been wearing a jockstrap, I suppose. I was standing behind him and didn't care to look.

"I haven't talked about it before because while he was politically important I didn't want to ruin his career. Now he's standing down so it doesn't matter."

A Downing Street spokesman described the story as "totally and utterly untrue", but Mr Sharrock stood by his version of events.

Last month newspapers published a photo of Tory leader David Cameron in a white tie and tailcoat during his own Oxford days, leading to him being labelled a "toff".

BBC reporter Michael Crick unearthed the photo of the Prime Minister to show that Mr Cameron is not the only party leader who might prefer to forget some of his youthful foibles and somewhat privileged background.

Mr Blair was privately educated at Fettes College in Edinburgh, where he was caned for persistently breaking school rules and was threatened with expulsion.

He was also arrested at Fettes after being mistaken for a burglar as he tried to climb back into his dormitory using a ladder, having found himself locked out after an illicit late-night excursion.