London (CNN) -- Britain's government has evidence Russia was involved in the mysterious poisoning death of former KGB agent Alexander Litvinenko, who was at that time working for British and Spanish intelligence services, a lawyer has said.
The allegation emerged at a hearing Thursday in London ahead of an inquest into Litvinenko's 2006 death that is due to start in May.
In Britain, an inquest is an inquiry held where a death is sudden or unexplained to establish the facts around it.
Litvinenko, a fierce critic of Russian President Vladimir Putin, came to Britain in 2000 after turning whistle-blower on the FSB, the KGB's successor.
He died at a London hospital November 23, 2006, after being poisoned by the radioactive material polonium-210. In a deathbed statement, he blamed Putin, an accusation the Kremlin has strongly denied.
The counsel to the inquest, Hugh Davies, said evidence provided by the UK government "does establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko."
The confidential government material does not, however, support the idea that the British government itself killed Litvinenko, he said. Nor does it suggest that fellow Kremlin critic Boris Berezovsky, the Spanish mafia or other organized crime groups are to blame, Davies added.
Prosecutors in London want Russia to extradite Andrei Lugovoi, suspected in the killing, but Russia has repeatedly refused to do so.
Lawyer Ben Emmerson, acting for Litvinenko's widow Marina, told the hearing that Litvinenko was working for the British intelligence service MI6 and had been tasked by MI6 with working also for the Spanish intelligence service as it investigated Russian mafia activities in Spain.
Litvinenko had an MI6 handler known only as "Martin," whom he would meet in central London, he said.
Payments, both from MI6 and from the Spanish intelligence services, were made directly into the joint bank account held by Litvinenko and his wife, Emmerson said.
While ill in hospital Litvinenko called Lugovoi about a planned trip together to Spain, a phone call that was witnessed by his wife, Emmerson said.
The men were both to provide intelligence to the Spanish prosecutor investigating Russian mafia links to the Kremlin and to Putin, he said.
Emmerson also questioned whether the British government did enough to protect Litvinenko from threats to his safety in light of his relationship with MI6.
Russia's Investigative Committee has confirmed that it wants to be involved in the process as an "interested party," the state-run RIA Novosti news agency said Friday.
If Russia becomes an interested party, its representatives will be allowed to cross-examine witnesses and study the evidence, it said.
A statement on the website of the Russian Embassy in London said the Investigative Committee's involvement would help the inquest "in securing the all-sided, comprehensive and objective consideration of the case."
The committee, a federal agency, is carrying out its own investigation into the circumstances of Litvinenko's death in Russia, the statement said.
Emmerson told the hearing, held before high court judge Sir Robert Owen, that he and Litvinenko's widow were keen for Russia to have interested party status.
Russia has not yet commented on the claims of evidence of its involvement in Litvinenko's death.