"Allegations that someone stole it during production are absolutely unfounded," said Sergei Kiriyenko, director of the nuclear agency Rosatom. "The controls are very tough."

Kiriyenko said Russia exports 8 grams of polonium-210 monthly, all of it to the United States. He said there had been no exports to Britain in five years.

Polonium-210 is highly toxic substance with 138 days half-life.