Transport secretary Lord Adonis has confirmed that airport body scans will be compulsory for passengers selected to undergo the procedure. Travellers who refuse will not be allowed to fly.
"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 said.
Heathrow and Manchester airports started using the scanners today, and others, including Birmingham, are set to follow later this month.
A public consultation on the use of the controversial technology, which produces effectively naked images, will follow.
"The Code will require airports to undertake scanning sensitively, having regard to the rights of passengers. 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."
Alex Deane, director of privacy news site Big Brother Watch, said: "What kind of a free society does the government think it is 'protecting' when it invades our privacy like this?
"When we are forced to expose ourselves at the airport in order to go on holiday, the terrorists have won."
Last week, Interpol chief Ronald Noble was reported as questioning the money and resources being ploughed into body scanners. He argued that better intelligence and information sharing between countries would better serve anti-terrorist efforts.
With £6.7m in initial funding, Mosa Meat could be the first company to offer lab-grown meat to the public
Manufacturing and finance jobs will be hit, but health and education can look forward to job creation, says PwC
US startups plan to modify existing jet engines, but are likely to fall foul of environmental legislation
The Brexit white paper "gets pretty close" to company desires, but there's still work to do