Airline passengers will have no right to refuse to go through a full-body search scanner when the devices are introduced at Heathrow airport next week, ministers have confirmed.

The option of having a full-body pat-down search instead, offered to passengers at US airports, will not be available despite warnings from the government's Equality and Human Rights Commission that the scanners, which reveal naked bodies, breach privacy rules under the Human Rights Act.

The transport minister Paul Clark told MPs a random selection of passengers would go through the new scanners at UK airports. The machines' introduction would be followed later this year by extra "trace" scanners, which can detect liquid explosives. A draft code of practice covering privacy and health issues is being discussed in Whitehall.

Clark dealt with concerns raised by the Commons home affairs select committee about the ability of airports abroad to upgrade their security to similar levels by indicating that extra support and help was under discussion.

Lord West, the counter-terrorism minister, told the MPs the government had firmly ruled out the introduction of "religious or ethnic profiling" into transport security. Instead, he said, airport security staff were being trained in "behavioural profiling", which meant spotting passengers who had paid cash, were travelling with only a book for luggage on a long-haul flight or were behaving erratically at the airport.

He said the decision to raise the terror threat level to "severe" – meaning an attack was highly likely but not imminent – had been taken by security service officials at the Joint Terrorism Analysis Centre last Friday.

The decision, thought to be based on an increase in intelligence traffic on threats from Yemen, was confirmed by the home secretary, Alan Johnson.

West refused to discuss the intelligence behind the decision, saying he was not going to jeopardise "getting the bastards".

The body scanner trials, which are due to start at Heathrow next week, will involve a machine that has spotted the type of concealed device used in the Detroit airline bombing attempt.

The airport's owner, BAA, is preparing to install a scanner in each of its five ­terminals. The trials will use two different technologies that see through passengers' clothing. One trial will involve "backscatter" technology, which exposes travellers to low-level x-rays. This is already in use at Manchester airport. Security staff at Manchester recently replicated the underwear bomb that Umar Farouk Abdulmutallab smuggled on to Northwest Airlines flight 253. The machine singled out elements of the fake weapon.

"We could see that there was something on the person that would have required a further search," said a spokesman for Manchester airport, whose machine requires passengers to stand between two Tardis-like blue boxes.

Under the new security regime, due next month, the suspect passenger would then be led away for a secondary examination that would include using chemical swabs to test for explosives. Pat-down searches of passengers and hand luggage inspections will also increase.

The second type of machine uses a "millimetre wave" system, which bounces radio waves off the human body to form a 3D image of the passenger. Both types of technology have raised privacy concerns owing to the graphic nature of the passenger images, with civil liberties campaigners calling the process "virtual strip-searching".

The Department for Transport has drawn up a preliminary code of conduct for using the machines, and it will follow some guidelines used in the US. These state that the security officer guiding the passenger through the machine never sees the image, and that the employee viewing the scan must be based away from the passenger, in a secure room. The two officers communicate with wireless headsets; and, once viewed, the scan cannot be saved, printed or transmitted.