Travellers at New York and Los Angeles airports will be among the first in the world to be searched using a new scanner which can see through clothes.

The millimetre wave imaging technology, which begins trials this week, creates an picture of the body which critics say amounts to a virtual strip search.

But security officials say it can show contours of the body and can pick up hidden items, such as guns or knives and may be more effective than a physical 'pat-down' in detecting contraband.

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After the normal airport routine of walking through metal detectors, some travellers will be selected at random to go through the scanning devices, which each cost £75,000. Passengers will walk into a large booth and the machine will beam electromagnetic waves on to the body, creating a three-dimensional image from reflected energy.

Security staff in a separate room will examine the image, which will later be erased from the system. Officials officials say this distance protects a person's privacy because, apart from the image, they are unable to see the people being examined. The passenger's face is blurred and the images are not stored.

Passengers may choose not to go through the scanner, but will then be subject to other screening, including pat-down searches.

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