Snap! Why public figures are continually getting caught out by Steve Back

Mark Thompson is latest victim of scourge of Downing Street

Once his long lens caused mild embarrassment by revealing the shapely legs of the future Princess Diana through her transparent summer skirt.

More recently, Steve Back has repeatedly found himself in a position to cause red faces of an altogether different magnitude by the simple means of standing outside No 10 with a camera.

The freelance press photographer this week added BBC director general Mark Thompson to the growing list of visitors to Downing Street who he has captured revealing sensitive – and sometimes top secret – information by leaving documents on display in apparently blissful ignorance of the fact that they are outside the world's most closely-scrutinised front door.

The risks run by public functionaries who arrive at No 10 without that piece of hi-tech counter-surveillance technology otherwise known as a cardboard folder were most dramatically revealed by Mr Back's lens last April when Scotland Yard's anti-terrorism commander was pictured carrying a briefing paper detailing an operation against an alleged al-Qa'ida cell. The blunder led to a news blackout while police hurriedly brought forward a series of raids in North-west England – and Assistant Commissioner Bob Quick's red-faced resignation.

A succession of ministers – ranging from former housing minister Caroline Flint who was seen carrying a memo laying bare official fears about the property crisis, to a secret election briefing sent to Lord Mandelson – have fallen foul of Downing Street's document snapper, who says his penchant for picturing personal papers comes despite repeated warnings to No 10.

Mr Back, 55, a national newspaper photographer for 25 years who is to be found in Downing Street's press pen most days, said his images of Mr Thompson carrying an internal BBC email detailing a defence of its coverage of spending cuts were just the latest proof of document-based carelessness by public figures.

He said: "It's astonishing really. These are people out in public with papers containing important information. They know that there is a photographer outside No 10 and yet they just don't cover them up. I must have spoken to the Downing Street press office a dozen times to say: 'For God's sake tell them to cover up their documents.'

"It is all down to digital technology. Images come out so sharp now that you can read detail that film cameras simply could not pick out. I would say I see a minister carrying readable documents once a month."

The fashion for inadvertent disclosures began in 2008 when Ms Flint marched up to No 10 displaying a candid memo warning of a five to 10 per cent drop in housing prices "at best" and admitting ministers "can't know how bad it will get".

Other caught with unsheathed papers by photographers include Lord Mandelson, who was pictured last September with a briefing on how to attack the Conservatives, and Hazel Blears, the former communities secretary, who was seen carrying details of the shake-up of MPs' expenses.

The Coalition Government is not immune. Andrew Mitchell, the International Development Secretary, and Climate Change Secretary Chris Huhne have both been pictured carrying easily-decipherable documents.

Mr Back, who took a famous photograph of Diana Spencer before her marriage showing a silhouette of her legs and underwear as she stood in the sun in a see-through skirt, points out the lengths to which he has gone to behave responsibly. He is, after all, a west London magistrate when not stood outside No 10 – and does not like being photographed himself.