Britain introduced body scanners at Heathrow airport on Monday, a measure rushed in after a failed attempt by a Muslim extremist with explosives strapped to his leg to bomb a US-bound passenger plane from Amsterdam.

The scanners, which see through clothes to produce an image of the body, have caused unease among human rights campaigners who fear an invasion of passengers' privacy as well as the disproportionate scrutiny of Muslim travellers by authorities.

"Given the current security threat level, the government believes it essential to start introducing scanners immediately," said Transport Secretary Andrew Adonis.

Britain raised its terrorism threat level to "severe", the second-highest level, on January 22, days before London was due to host two international conferences on Yemen and Afghanistan. The conferences took place last week without any security incident.

The British government has been particularly concerned about the botched attempt by suspect Umar Farouk Abdulmutallab, a Nigerian national, to blow up a plane bound for Detroit on December 25 because he was a student in London between 2005 and 2008.

Abdulmutallab boarded the US flight at Amsterdam's Schiphol airport, which already has 15 body scanners and plans to install more. France and Italy have also signalled they would start using the devices at their airports.

In a statement, Adonis said airports at Heathrow and Manchester, northern England, were the first required to use the scanners and others would follow. Scanners will be introduced at Birmingham airport (in central England) this month.

"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.

An interim code of practice for security staff stipulated that passengers should not be selected for scanning on the basis of gender, age, race or ethnic origin, Adonis said.

He added that the government would launch a public consultation on the rules that should be applied in the use of scanners, with a view to producing a final code of practice.

The merits and uses of body scanners have been vigorously debated in Europe since the failed Christmas Day bombing.

The European Union's new transport chief, Siim Kallas, said last month member states should refrain from using the devices until the bloc agrees on rules to protect privacy and health. But the bloc's anti-terrorism chief, Gilles de Kerchove, said days earlier that all EU countries should introduce them.-Reuters