LONDON — Alexander Litvinenko, the former Russian agent turned Kremlin critic, was a “registered and paid” agent working for Britain’s foreign-intelligence agency when he died after being poisoned, a lawyer representing his widow told an official hearing Thursday. Another lawyer said Britain has evidence the Russian government was behind Litvinenko’s death.

Litvinenko, 43, died in November 2006 after drinking tea laced with the rare radioactive isotope polonium-210 at a London hotel. Six years later, British authorities are reopening investigations into his death.

On his deathbed, the former Russian FSB agent accused the Kremlin — specifically Russian President Vladimir Putin — of being behind his killing, and his family has long demanded Russian authorities be held accountable.

The case has strained relations between Britain and Russia, which denies poisoning the former Russian agent.

Thursday’s session aimed to set the scope of a public inquest into Litvinenko’s death. Judge Robert Owen said the inquest is expected to start in May.

Lawyer Ben Emmerson, representing Litvinenko’s widow, Marina, said that at the time of his death, Litvinenko was working for Britain’s MI6 spy agency and had been told to help Spanish intelligence investigate the Russian mafia. The British probe must consider if MI6 failed to properly assess the risks before sending the agent on his assignment, Emmerson said.

He said Litvinenko worked for MI6 for several years and often met in London with a hand­ler from the agency known only as “Martin.” Payments from the British and Spanish intelligence agencies were made to a joint bank account held by the agent and his wife, Emmerson said.

Shortly before his death, Litvinenko was due to travel to Spain with former KGB bodyguard Andrey Lugovoi to provide intelligence to Spanish authorities, Emmerson told the hearing. He said he was basing his claims on information Marina Litvinenko gave to British police.

Britain has accused Lugovoi and another Russian, Dmitry Kovtun, of killing Litvinenko. Russia has refused to hand them over, and Lugovoi has publicly denied involvement in the death.

Lawyer Neil Garnham, representing Britain’s Home Office, told the hearing he could not confirm or deny Litvinenko was employed by British intelligence.

Meanwhile, Hugh Davies, the lawyer who advises the coroner in the inquest, told the hearing that a “high-level assessment” of confidential material provided by the British government established a case for the Russian state’s culpability in Litvinenko’s poisoning.

The review of the material ruled out responsibility of the British government in the death, he said.