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Mr Stringer said there were various techniques for protecting people’s privacy.

“We can solve the modesty issue by overlaying the body with graphics except for the area which causes concern.

“The computer can also be set only to show those people who are carrying something suspicious.”

Mr Stringer said that the devices could either be deployed covertly or made prominent at station entrances as a visible deterrent.

Dummy devices could be installed at some stations to reduce the overall cost.

As with speed cameras, people passing would not know which ones were live.

QinetiQ has also combined the devices with software which detects anomalous behaviour, such as people changing direction inside mainline or Underground stations. Covert studies of terrorists have detected typical patterns of movement as they carry out reconnaissance or seek the best position for their attack.

Mr Baum said vigilance by passengers would play a greater role than any technology in detecting terrorists.

Even if the Tube and mainline stations were all covered by scanners, it would be impossible to protect all 8,000 buses in London.

After last year’s Madrid train bombings, there was a high degree of awareness in London of the risk from suspect packages.

But the number of alerts had dwindled in recent months and Tube managers believe that many passengers had become reluctant to delay services with what they assumed would be false alarms.

Some staff had also begun to take the threat less seriously.

In February, a woman was reprimanded by platform staff after blocking the door with her foot when she saw an unattended bag on a train at Bank station on the Northern Line.

• The return to service on London Underground was delayed last night after Tube drivers refused to carry out security inspections on trains. The drivers argued that police or army officers should give the all-clear before services could resume. The Rail Maritime and Transport union said that drivers had been asked to carry out security inspections on Tube trains, which was “completely unacceptable”.

  Bob Crow, the RMT union’s general secretary, said: “We don’t believe drivers should be checking trains until police or army officers have given the all clear.

  “We obviously apologise for any delays this will cause, but the threat is too serious for such crucial inspections to be done by untrained staff.”

  The union said that it had been kept in the dark over security plans and called for a review of the arrangements that would involve the trade unions.

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