The strict rules were confirmed as scanners were introduced at Heathrow and Manchester airport as security was stepped up following the attempt to bring down a transatlantic flight on Christmas Day.
All passengers, even children, face potential selection, the Department for Transport said.
The scanners, which are designed to detect explosives, at Heathrow’s Terminal 4 and Manchester are part of a programme which will eventually be rolled out to all major airports.
At the same time Heathrow has been training staff how to look out for unusual behaviour as passengers pass through the airport to decide who should face additional screening.
Such behaviour could include being nervous or agitated when passing through the terminal.
In addition intelligence information will also be used to identify which passengers should be subject to further checks, which could include scanning or further questioning.
Terminal 4 has been chosen for the first of Heathrow’s scanners because it is used by a number of transatlantic airlines.
The scanner will be placed in a purpose-built cubicle where passengers who are selected for screening will be taken after passing through the metal-detecting arch.
It will take about 15 seconds to scan an individual. This will be done the passenger reaches the departure lounge.
Heathrow declined to say how many passengers will be expected to go through a body scanner a day.
Lord Adonis, the Transport Secretary, said only a small proportion of passengers will be selected for scanning.
He added that a code of practice meant that nobody can be selected on the basis of race, age or gender. The assurances are designed to avoid claims that Asians could be more likely to be picked out than others.
Other safeguards announced by the Government include giving passengers the right to demand that the individual inspecting the scanner image on a screen is the same sex.
In addition the security officer inspecting the image will not see the passenger and all images will be destroyed after the individual has walked away from the scanner.