The widow of former Russian spy Alexander Litvinenko has told an inquiry into his radiation poisoning death that her husband had suspected Vladimir Putin of being involved in "criminal conduct".

She also said Mr Litvinenko met with Mr Putin in 1998, when the now Kremlin leader was head of the Russian secret service, to expose an alleged plot he had discovered to kill the dissident oligarch Boris Berezovsky.

Marina Litvinenko said Mr Putin had done nothing and that shortly after raising his concerns, Mr Litvinenko himself, known to his family as Sasha, had come under investigation.

"Sasha said it was not a productive meeting at all and he didn't believe there will be any action," she told London's High Court.

A former KGB spy, Mr Putin started out in post-Soviet politics as deputy mayor of Saint Petersburg in the early 1990s before going on to head up the FSB agency.

"On his position of deputy mayor of St Petersburg, Sasha believed [Mr Putin] was involved in some criminal conduct," she said, adding the city was the crime capital of Russia at that time.

Leaked cables show US diplomats viewed Mr Putin as a leader who ruled by allowing crooked spies and corrupt officials to siphon off cash from Russia, the world's biggest energy producer.

The Kremlin has repeatedly dismissed the accusation as nonsense.

In late 1998, Mr Litvinenko fronted a news conference Mr Berezovsky had organised with a number of other FSB agents, some masked, to denounce corruption in the FSB.

In 1999 Mr Litvinenko was charged and detained before being cleared of assaulting a suspect. He was immediately re-arrested and accused of theft.

That case was dropped but he faced further allegations in early 2000 shortly before he fled Russia on a false Georgian passport, his widow said.

In 2013, Mr Berezovsky was found dead on the bathroom floor of his home in southern England with a ligature around his neck.

"It was Putin's fault why Sasha was in prison," Ms Litvinenko said.

"Sasha said ... 'if they [arrest] me, I will never be able to go out from prison, they will kill me'."

Ms Litvinenko said her husband believed she and their son were also in danger.

The family was granted asylum in Britain in May 2001, given new names and gained British citizenship weeks before Mr Litvinenko's death.

"He was very proud to be British," Ms Litvinenko said.

On his hospital bed in London in 2006, Litvinenko accused Vladimir Putin of ordering his killing. ( Reuters )

Mr Litvinenko was 43 when he died in 2006 after drinking tea poisoned with the rare radioactive isotope polonium-210, which British police believe he was given by two Russians.

The main suspects, Andrei Lugovoi and Dmitri Kovtun, deny involvement and Russia has refused to extradite them to face trial.

The inquiry, which opened last week, has heard Mr Litvinenko told police that Mr Putin, who served as a KGB spy in East Germany, ordered his killing.

Russia has rejected any role in Mr Litvinenko's killing.

Ms Litvinenko also told the judge-led inquiry that Mr Litvinenko had been working for Britain's MI6 foreign intelligence service at the time of his death.

She said that her husband was not "employed" as an MI6 agent but did "consult" for both the British and Spanish intelligence services.

Ms Litvinenko said his task was to provide information about Russian criminal gangs, not to hand over names of sleeper agents.

Asked if he gave anything to Britain that Moscow might want kept secret, she said: "If a Russian government [was] connected to organised crime, yes."

The inquiry hearings began last week and are expected to last another two months, with a report due to be published by the end of the year.

Reuters/AFP