Passengers using Heathrow and Manchester airports will not be allowed to board their flights if they refuse to undergo a full-body scan. But in a bid to allay privacy fears, the Transport Secretary Lord Adonis promised that the images are deleted “immediately”.

Despite opposition from civil liberties campaigners, the advanced imaging technology (AIT) scanners are now in operation at the two airports, and will be followed by Birmingham airport at the end of this month, followed by a nationwide roll-out.

The tougher security measures follow the attempted bomb attack aboard a US plane on Christmas Day. A Nigerian suspected member of Al Qaeda is accused of trying to detonate a bomb hidden in his underpants as his flight was about to land in Detroit.

Lord Adonis, the Transport Secretary, said that the scanners would help security staff to detect dangerous items, including explosives.

“At the moment, people accept that they are subject to a pat-down search when they have gone through the metal detector,’’ he told BBC Breakfast.

“A pat-down search is a pretty intrusive procedure but people accept that because it is important that we do detect whether there are weapons or other powerful substances that people may be carrying.”

“A body scanner is in the same category - it is very important to stress that the images which are captured by body scanners are immediately deleted after the passenger has gone through the body scanner.

“Given the current security threat level, the Government believes it essential to start introducing scanners immediately. However I wish to consult widely on the long-term regime for their use, taking full account of the experience of the initial deployment.”

He added: “In the immediate future, only a small proportion of airline passengers will be selected for scanning. If a passenger is selected for scanning and declines, they will not be permitted to fly.”

He also said that passengers would not be selected for the scans “on the basis of personal characteristics”.

Ian Hutcheson, BAA Security Director said: “The introduction of full-body scanners and other technology is one significant step towards a more robust defence against the changing and unpredictable threat posed by terrorists.”

“However it is important that, as a country, we make better use of the intelligence available to industry and Government and continue to promote the close assessment of passengers’ behaviour.  Only by doing so will we build a robust security system that is dynamic enough to respond quickly and effectively to the emerging threat.”

BAA undertook the first ever trial of body-scanning technology in 1991.  A second trial took place at Heathrow from April 2004 until June 2008.

Sarah Barrett, head of customer experience at Manchester, told The Telegraph that passengers had “privacy concerns” but emphasised that the airport had put in strict procedures.

“It will enhance security for everyone, which can only be a good thing, without compromising people’s privacy,” she said.

“The image generated by the body scanner cannot be stored or captured, nor can security officers viewing the images recognise people. Contrary to reports, the equipment does not allow security staff to see passengers naked.”