Murdered former spy Alexander Litvinenko worked for MI6 as a double agent and was killed as an act of "state sponsored assassination" by Russia, it was claimed at a pre-inquest hearing on Thursday.
According to lawyers for the inquest the 46-year-old former KGB agent was working for British spy when he died as well as the Spanish intelligence services.
The 43-year-old was poisoned in November 2006 in a meeting at a London hotel with polonium-210 and died three weeks later.
Russia has refused to extradite prime suspects Andrei Lugovoy, who is a Russian MP, and Dmitry Kovtur.
Prosecutors named Lugovoy as the main suspect in the case but Russia has refused to extradite him to the UK for questioning.
Hugh Davies, counsel to the inquest into Mr Litvinenko's death, said assessments of confidential material submitted by the British Government had "established a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko".
Ben Emmerson QC, representing Mr Litvinenko's wife Marina, said the inquest should also consider whether MI6 failed in its duty to protect against a "real and immediate risk to life".
"On the view of the counsel to the inquest there is a prima facie case of state sponsored assassination on the evidence they have seen," he said, according to the Daily Mail.
Mr Litvinenko had been hired by MI6 for a number of years and was working with the Spanish secret service investigating the Russian mafia shortly before his death, a pre-inquest review at Camden Town Hall, in London, heard.
He would regularly meet with an MI6 handler, named only as Martin, in central London and was paid by both the British and Spanish secret services into a joint bank account he held with his wife, the hearing was told.
The Russian Federation has now indicated its wish to become an interested party in the inquest, which is to be held on May 1.
After the hearing Mrs Litvinenko said she was pleased the alleged involvement of the Russian government in her husband's murder would now be considered by the inquest.
"We've been saying this many times but this is the first time this question has been raised in court," she said.
"I appreciate all that was done today and I'm looking forward to any decision which will be taken by the coroner after today's hearing."