Dr Pat Troop, chief executive of the Health Protection Agency, has said Mr Litvinenko's death was an "unprecedented event in the UK" and that he had "apparently been poisoned by a type of radiation".
A large quantity of alpha radiation - probably from a substance called polonium 210 - was detected in Mr Litvinenko's urine, Roger Cox, director of the HPA's centre for radiation, chemicals and environmental hazards, said.
More on this story as we get it.