Litvinenko, a former agent for the Soviet KGB, was poisoned in 2006 by drinking radioactive polonium-210 from a tea cup while meeting with former colleagues at a London hotel. He died at University College Hospital three weeks later.
At the hearing Thursday, Ben Emmerson, a barrister for Litvinenko's wife Marina, said the Russian was a paid informant for British intelligence, which was investigating Russian organized crime and possible links to President Vladimir Putin, The Daily Telegraph reported.
Emmerson said Litvinenko had an MI6 handler called "Martin" and he worked with former KGB bodyguard Andrei Lugovoi. Lugovoi, one of two suspects in Litvinenko's poisoning, has denied any involvement in his death.
Lugovoi was later elected to Russia's Duma, the lower House of Parliament, and cannot be extradited, effectively giving him immunity from prosecution.
Inquest counsel Hugh Davies said evidence presented at the preliminary inquiry "does establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko," The Guardian reported.
Emmerson testified Litvinenko's only contact with "Martin" was by a dedicated phone and money was regularly deposited in the couple's joint bank account, the Telegraph said. Litvinenko reportedly was encouraged by MI6 to supply intelligence on the Russian mafia to a Spanish prosecutor and to Spain's intelligence service.
"There is no question that Alexander's involvement with Martin and MI6 was true," Emmerson said.
Russia's Investigatory Committee has asked to join the inquest hearing, which is expected to hear more evidence May 1.
Notable deaths of 2014 [PHOTOS]