A leading rabbi says that "naked" X-ray body scanners at airports are "immodest" and pose a halachic problem for religious Jewish women.

Dayan Yisroel Yaakov Lichtenstein, head of the Federation Beth Din, spoke as the government announced that passengers could not opt out of a scan if requested by security staff.

Transport Secretary Justine Greening said that the full-body scanners, which see through clothes and produce an image of skin and genitalia, should be rolled out after successful trials, currently under way in Heathrow, Gatwick and Manchester airports.

She said: "Most responses to our consultation expressed discomfort with the idea of having an image of their body captured for analysis and they indicated that, if selected for a security scan, they would prefer to opt for an alternative method of screening.

"I have considered this carefully. However, I have decided against it, on security, operational and privacy grounds." Ms Greening said passengers who refused a scan would not be allowed to fly.

But Dayan Lichtenstein said the Torah was particularly concerned about the dignity of women and that Orthodox women must request female security staff to view their image.

He said: "A man watching the scan of a woman is immodest in halachah. If the authorities say that a person has no right to insist on same-sex viewing, then the community should mount a legal challenge against it. But I can't imagine any normal authority would argue that men have a right to view images of women. A person has a right to dignity."

The Department for Transport later clarified that it would be mandatory for security staff to uphold a passenger request for same-sex viewing.

Fears could be allayed by new technology being tested at Heathrow Airport in which computers view images.

But Manchester Airport, the only place to have X-ray scanners across all terminals, said human viewing would remain its policy. A spokesman said: "When someone stands in these new scanners with wet clothes, for example, you can't actually see the image to identify anything. We believe they don't work and have, therefore, been trialling the X-ray scanners."

Manchester Beth Din's Dayan Yitzchok Berger, however, believed the scanners posed no halachic problem.

He said: "It seems there is a real danger of terrorists, and in the same way you go to a doctor and have yourself examined to save your health, never mind your life, one could use these machines.

"I've been through one myself at Manchester Airport two weeks ago. I would love to see an image of how good the machine is."