I felt fat and naked with 'virtual strip search'

Nicknamed "nude" scanners, they work by bouncing tiny microwaves off passengers who stand in an archway or booth. A three-dimensional image is then created, producing a very detailed outline of what lies beneath clothes – metals and plastics are easily identifiable.

The scanners are controversial because the image clearly shows breasts and genitals. Critics argue they are an invasion of privacy akin to a "virtual strip search".

In order to allay these fears, the technology is being tweaked to blur the face of the passenger or obscure the individual's genitals – known as "fig leaf" software.

The procedure can also be arranged so that the machine operators do not see the subjects, the images instead being analysed by staff working elsewhere in the airport.

Concern over the graphic nature of the images produced was heightened in 2003 when Professor Susan Hallowell, an official at the American Transportation Department, allowed a scanner image of herself to be distributed.

She spoke of how the scanner made her look "fat and naked", but the image also identified a fake firearm she was concealing.

Use of the 100,000 machines, which are a technological alternative to pat-down searches, is voluntary at Manchester Airport during the trial there because of privacy concerns.

When the trial started, Manchester Airport's head of customer experience, Sarah Barrett, said: "Our passengers tell us that they don't like being patted down … but they understand that it's a necessary part of keeping them safe."

The number of people volunteering to be scanned has increased from 75 per cent to 92 per cent after the Detroit incident, the airport said.