Electromagnetic waves are beamed onto passengers to create a 3D image
Some passengers at Heathrow and Manchester airports will have to go through full body scanners before boarding their flights under new rules.
It is now compulsory for people selected for a scan to take part, or they will not be allowed to fly.
The new security rules have been introduced following the attempt to blow up a plane over Detroit on Christmas Day.
There have been concerns the scanners breach passengers' rights to privacy.
Transport Secretary Lord Adonis said in the immediate future only a small proportion of airline passengers would be selected for scanning.
In a written statement to the House of Commons, he said: "If a passenger is selected for scanning, and declines, they will not be permitted to fly."
The scanners were introduced at the two airports on Monday and will be installed at Birmingham later in the month.
The machine has been used at Manchester Airport's Terminal 2 since October, with additional scanners planned for Terminals 1 and 3 by the end of February.
The Department of Transport has published an interim code of practice covering privacy, health and safety, data protection and equality issues.
"The code will require airports to undertake scanning sensitively, having regard to the rights of passengers," Lord Adonis said.
Manchester Airport said it had put strict procedures in place to protect the privacy of passengers.
Its head of customer experience, Sarah Barrett, said: "It will enhance security for everyone, which can only be a good thing, without compromising people's privacy.
"The image generated by the body scanner cannot be stored or captured nor can security officers viewing the images recognise people."
The equipment does not allow security staff to see passengers naked, she added.
A rule which meant under 18s were not allowed to participate in the body scanner trial has been overturned by the government.
The introduction of body scanners has sparked a wide debate, and even the home secretary has admitted it will not be a "magic bullet".
Umar Farouk Abdulmutallab is accused of trying to detonate a bomb on a flight from Amsterdam as it was about to land in Detroit.