Electromagnetic waves are beamed onto passengers to create a 3-D image
The UK's equality watchdog has written to the home secretary expressing concerns about plans to use body scanners at airports.
The Equality and Human Rights Commission (EHRC) said the devices risked breaching an individual's right to privacy under the Human Rights Act.
They are being introduced in response to the alleged attempt to blow up an American plane on 25 December.
Ministers have said protecting people's life and liberty is paramount.
The prime minister has pledged to introduce full body scanners at British airports, and they are due to be in operation at Heathrow Airport by the end of this month.
They produce "naked" images of passengers, and civil rights groups warn they could generate illegal images of children and images of celebrities that could be leaked online.
In response to such fears, the Department of Transport said it was developing a staff code of practice for airport body scanners.
In addition to body scanners, the government is also considering "additional targeted passenger profiling".
In its letter, the EHRC calls on Home Secretary Alan Johnson to set out in detail the justification for bringing in body scanners, and clarify what safeguards will be put in place.
The watchdog has said the proposals are likely to have a negative impact on privacy, especially in relation to certain groups such as disabled people, the elderly, children and the transgendered community.
It also wants to see the evidence for the profiling of air passengers.
John Wadham, group director legal at the EHRC, said the commission accepted the government had a responsibility to protect air travellers.
"The right to life is the ultimate human right and we support the government reviewing security in the light of recent alleged terrorist activity," he said.
"However, the government needs to ensure that measures to protect this right also take into account the need to be proportionate in its counter-terrorism proposals and ensure that they are justified by evidence and effectiveness."
Privacy campaigners welcomed the EHRC's move.
Dylan Sharpe, campaign director of Big Brother Watch, said the government had not considered privacy in its "desperation to be seen to be doing something".
"They are another intrusion into our privacy in the name of protection, yet we know that they are not fail-safe and could see airport authorities becoming reliant on a deeply flawed method of detection," he added.
The introduction of body scanners has sparked a wide debate, and even the home secretary has admitted it will not be a "magic bullet".
Earlier this month, he said: "It is clear that no one measure will be enough to defeat inventive and determined terrorists and there is no single technology which we can guarantee will be 100% effective against such attacks."
Mr Johnson also told the Commons there was only a 50 to 60% chance that a body scanner would have detected bomb materials allegedly carried by Umar Farouk Abdulmutallab.
He said the government would be "mindful of civil liberties concerns but conscious of our overriding obligations to protect people's life and liberty".
Mr Abdulmutallab is accused of trying to detonate a bomb on a flight from Amsterdam as it was about to land in Detroit.