The device at Terminal 4 produces a “naked” image of passengers by bouncing X-rays off their skin, enabling staff instantly to spot any hidden weapons or explosives.

But the graphic nature of the black and white images it generates — including revealing outlines of men and women — has raised concerns about privacy both among travellers and aviation authorities.

In America, transport officials are refusing to deploy the device until it can be further refined to “mask” passengers’ modesty.

The Terminal 4 trial — being conducted jointly by the British Airports Authority and the Department for Transport — became fully operational last month and is intended to run until the end of the year. Its deployment has not been reported until now since new security measures at airports are not normally publicised.

If the new body scanner is able to cope with large volumes of passengers, improves detection rates and, crucially, receives public acceptance, it is likely to be rolled out across all Britain’s airports.

At Heathrow, passengers are picked to go through the body scanner on a random and voluntary basis. Those who refuse are subjected to an automatic hand search.

The scanner, which resembles a tall, grey filing cabinet, operates in a curtained area and passengers are asked to stand in front of it, adopting several poses, for their “naked” image to be registered. Once checked, the images are immediately erased.

Security officials claim it is a far more effective way of countering potential terrorists because it detects the outline of any solid object — such as plastic explosives or ceramic knives — which conventional metal detectors would miss.

Managers at Heathrow also say the new technology does away with the need to subject passengers to potentially intrusive hand searches. However, travellers who have been screened — and have asked to see the images — have been surprised by their clarity.

“I was quite shocked by what I saw,” said Gary Cook, 40, a graphic designer from Shaftesbury, Dorset. “I felt a bit embarrassed looking at the image.

A female passenger, who did not want to be named, said: “It was really horrible. It doesn’t leave much to the imagination because you’re virtually naked, but I guess it’s less intrusive than being hand searched.”

In a similar trial at Orlando international airport in Florida in 2002, passengers were shown a dummy image before going through, and at least a quarter of them refused to volunteer.

In America last year, Susan Hallowell, director of the US Transportation Security Administration’s (TSA) security laboratory, showed off her own x-ray image to demonstrate the technology to reporters.

“It basically makes you look fat and naked, but you see all this stuff,” said Hallowell, who had deliberately hidden a gun and a bomb under her clothes.

The TSA has decided not to deploy the device at American airports until manufacturers can develop an electronic means of masking sensitive body parts.

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