In a document published earlier this month, the Department of Homeland Security outlined an update to the Advanced Imagery Technology protocols used by the TSA at US airports, adding a clause which allows officers to insist travelers go through the controversial machines.

Previously, though the body scanners were present at many airports across the country, travelers were free to opt-out of the process. Billed as a privacy consideration, it meant a physical screening was mandatory, but alleviated concerns held by some that the technology could "see them naked" and store photographs of that.

Now, though, that option is being diluted, though not completely retired.

"TSA is updating the AIT PIA to reflect a change to the operating protocol regarding the ability of individuals to opt opt-out of AIT screening in favor of physical screening," the DHS writes. "While passengers may generally decline AIT screening in favor of physical screening, TSA may direct mandatory AIT screening for some passengers."

No more detailed explanation for the change is given. However, it seems likely that the scanners' ability to single out metallic objects hidden around the body - and that might have been missed by a physical search from a TSA agent - is seen as invaluable for whoever security services believe presents a greater-than-normal risk.