X-ray cameras that would "undress" passers-by in a bid to thwart terrorists concealing weapons, could be coming to a street near you, according to reports. Aside from the obvious privacy issues, would such a plan work?

Leaked documents said to have been drawn up by the Home Office and seen by the Sun newspaper say cameras which can see through clothes could be built into lamp posts to "trap terror suspects".

While Home Secretary John Reid has denied knowledge of the plans, the technology is not dissimilar to that already found in some UK airports. Currently, air security officials pick out individuals to stand in a booth while three pictures are taken of the person in slightly different positions.

Within seconds, an X-ray scanner produces an image of the body, minus the clothes. What shows up is the naked human form and anything that may be concealed on the person, such as coins, a gun or drugs.

There are other variations on the X-ray technology. Millimetre wave machines give more of a three-dimensional image, while terahertz radiation also penetrates clothing.

A one-month trial at London's Paddington rail station involved a millimetre wave scanner. A portable version - an "electronic wand" - was trialled last year at London's Canary Wharf and Greenford Underground stations.

No decision has been made about wider implementation, according to a Department of Transport spokesman, who says the challenges are being evaluated.

But security expert Bob Ayers, of Chatham House, believes putting an X-ray lens on a lamppost poses all sorts of resource questions.

"Some guy walks past and his picture is beamed back to a control room to say that something is under his jacket. What do you do? Despatch a police car to hunt him down and frisk him?

"The real question is not whether the technology can see something under the clothing. It's how you respond to it when the technology says there's something unusual.

"Do you have police strolling down each street, ready to ask people what they have under their jacket?"

Privacy

The manpower cost would be "astronomical", he says, and CCTV would be required to match a description to the suspicious image.

"If you don't pick them up in minute or two, he's gone. What good does it do for you to know that at 11am this morning a guy walked down Victoria Street with a gun in his jacket?"

Besides, there could be problems distinguishing a money bag from a bomb strapped at the waist. But privacy should not be a concern because there is only a shadowy outline of the body, says Mr Ayers.

Professor Paul Wilkinson, a terrorism expert, is also doubtful. There would be a huge installation and maintenance cost, he says, plus the risk of antagonising ordinary citizens.

"The practicalities of these things working, if sufficient light, is in no doubt. The questions are when is this a useful addition to security and when does it become unduly intrusive and worrying to the public?"

What works for airline security, where passengers expect thorough checks, would not necessarily be tolerated when walking down the street, he adds.

Mr Ayers believes the best use of this technology is in a captured space, like at airport security or in a bus depot.

A spokesman for Qinetiq, one of the first firms to develop millimetre wave machines, says there have been successful uses for them.

"The Immigration Service has about eight or nine deployed around Europe, to see lorries crossing the Channel or at sea crossings into the UK.

"Using this technology alongside complementary technology such as CO2 monitors, they have detected thousands of people stashed away on lorries."

He adds that the US military are trialling millimetre wave machines at military checkpoints to combat the threat of suicide bombers. The use of cables mean they can be operated from any distance.