Health officials have drawn parallels between the murder of the former KGB colonel Alexander Litvinenko in London and the tragedy of the Chernobyl blast.
"This situation is utterly unprecedented," Bernie Wilkins, an environmental assessment officer for the Health Protection Agency told the Guardian. "The last radioactivity incident of this magnitude occurred when the radiation plume from Chernobyl swept over Britain in 1986."
"However, that kind of contamination was relatively easy to detect. The radioactivity involved had been sprayed out over the environment. This time we are trying to find out if a person has ingested a very small amount of an alpha-emitting isotope. That will take a lot more effort."
The April 26, 1986, explosion at Chernobyl's reactor No 4 spewed radiation across much of northern Europe over a 10-day period.
Official UN figures predicted up to 9,000 Chernobyl-related cancer deaths. But a Greenpeace report released last week estimated a figure of 93,000. Greenpeace said other illnesses could bring the toll up to 200,000.