The Kremlin has dismissed as 'sheer nonsense' claims it was involved in the poisoning of a former KGB colonel living in Britain.
The denial came as the condition of 43-year-old Alexander Litvinenko deteriorated and he was returned to intensive care in a London hospital.
A clinical toxicologist has said he had been poisoned with a potentially lethal dose of the metal, thallium.
Mr Litvinenko, who is a vocal critic of Russian President Vladimir Putin, fell ill at the start of this month after meeting a contact at a London restaurant.
He met a man who claimed to have information about the murder of the Russian journalist, Anna Politkovskaya, in October.
Police in Britain have confirmed they are investigating the incident.
Mr Litvinenko is reported to be under armed guard at the hospital.
