Passengers boarding London's Heathrow Express at Paddington are advised to allow a little more time for their journey since they may find themselves on the receiving end of a four-week, £500k trial of a US-built body scanner.

According to the BBC, passengers will be selected at random to enter a booth, raise their arms above their head and get scanned. Their luggage is probed separately by conventional X-ray machine. The body scanner is a millimeter wave machine - aka a "see through clothes scanner" - made by Santa Clara, California-based Safeview Inc and operated by Surrey-based Airlock Aviation Ltd.

There will reportedly be further tests at other locations, but the government has already ruled out blanket airport-style security on London's transport system on the perfectly reasonable grounds that it is completely impractical. The idea is deterrence, as Blair spokesman Tom Kelly put it: "We have to do everything we can to ensure that people know that there are security operations in place. If people believe that there is a chance they will be searched, that enhances security."

In reality, the Paddington test is purely to try out the technology, and is not intended as a real security screening exercise. ®