The US government is removing full-body X-ray scanners from seven major airports and replacing them with a different type of machine that produces a cartoon-like outline instead of the naked images that have been compared to a virtual strip search.

Transportation authorities say they are making the switch to speed up lines at crowded airports, not to ease passenger privacy concerns.

Civil liberties groups hope the change signals the old equipment will be scrapped.

“Hopefully this represents the beginning of a phase-out of the X-ray-type scanners, which are more privacy intrusive and continue to be surrounded by health questions,” said Jay Stanley, a privacy expert at the American Civil Liberties Union.

The old machines are being moved to smaller airports.

In the two years since they first appeared at US airports, the “backscatter” model of scanner has been the focus of protests and lawsuits because it uses X-rays to peer beneath travellers’ clothing.

Legislation approved in February gave transport authorities until June next year to get rid of the X-ray scanners or upgrade them with software that produces only a generic outline of the human form, not a blurry naked image. The agency, however, has the authority to grant itself extensions, and the current deadline is May 31, 2013.

Besides eliminating privacy concerns, new millimetre-wave scanners require fewer people to operate, take up less space in crowded security zones and complete a scan in less than two seconds, allowing screening lines to move faster.

“It’s all done automatically to look for threats, so you don’t have anybody in a back room that has to look at the imaging,” said Doug McMakin, who led the team that developed the technology.

In addition to speed and space advantages, the millimetre-wave technology does not produce the ionising radiation that has led to safety concerns with the X-ray machines.