Putin 'probably approved' radiation poisoning of ex-spy in Britain: UK investigators
Russian President Vladimir Putin attends a press conference in Prokhorovka, on July 12, 2013. (AFP)

Russian President Vladimir Putin "probably approved" the killing of ex-spy Alexander Litvinenko in London, a British inquiry into his agonizing death by radiation poisoning found Thursday.

Litvinenko, a prominent Kremlin critic, died three weeks after drinking tea laced with radioactive polonium at an upmarket London hotel in 2006.

Two Russians, Andrei Lugovoi and Dmitri Kovtun, were identified by British police as prime suspects but attempts to extradite the pair have failed.

The findings of the 300-page report pile pressure on Britain to act against Russia in response. Home Secretary Theresa May is due to outline the government reaction in a statement to parliament later on Thursday.

"The FSB operation to kill Litvinenko was probably approved by Mr (Nikolai) Patrushev and also by President Putin," the report said.

Patrushev is a former director of the FSB, the successor organisation to the Soviet-era KGB spy agency, and has been a key security minister since 2008.

"I am sure that Mr Lugovoi and Mr Kovtun placed the polonium-210 in the teapot at the Pine Bar on November 1, 2006," judge Robert Owen, the inquiry's chairman, said in the report.

Russia dismissed the findings, calling the inquiry "politically motivated."

"We had no reason to expect that the final findings of the politically motivated and extremely non-transparent process... would suddenly become objective and unbiased," foreign ministry spokeswoman Maria Zakharova said in a statement.

Lugovoi, now a lawmaker in Russia, described it as "absurd."

'Acting for a state body'

He accused London of "tunnel thinking" and an unwillingness to establish the true cause of Litvinenko's death in comments to news agency Interfax.

But Litvinenko's widow Marina, who spent years pushing for a public inquiry to be held, urged Britain to impose sanctions against Russia and a travel ban on Putin.

"I'm very pleased that the words my husband spoke on his deathbed when he accused Mr Putin of his murder have been proved true in an English court," she told reporters.

Litvinenko, 43, was poisoned in a bar at London's Millennium Hotel by a cup of tea poisoned with polonium-210 -- an extremely expensive radioactive isotope only available in closed nuclear facilities.

"The fact that Mr Litvinenko was poisoned with polonium-210 that had been manufactured in a nuclear reactor suggests that Mr Lugovoi and Mr Kovtun were acting for a state body rather than say a criminal organisation," the inquiry report said.

Inquiry chairman Robert Owen said that there was "no evidence" to suggest that either Lugovoi or Kovtun had any personal reason to kill Litvinenko and they were likely to be acting under FSB direction.

Litvinenko, an ex-KGB agent turned freelance investigator who also worked for British intelligence, accused Putin of ordering his killing in a statement before he died in agony three weeks later on November 23, 2006.

Owen said there was "undoubtedly a personal dimension to the antagonism" between Putin and Litvinenko, citing "repeated highly personal attacks" which the former spy made on the Russian president.

Litvinenko's killing caused public outrage in Britain after radioactive traces were found at various sites around London. It was dubbed by the media as the world's first act of "nuclear terrorism".

Britain's government announced the inquiry in July 2014, just days after the downing of a Malaysian passenger jet over eastern Ukraine -- a tragedy blamed on Russia's involvement in the conflict in the region -- in what was seen as a way of punishing Russia.

Litvinenko had fled Russia in 2000 and was granted asylum in Britain, later becoming a British citizen and converting to Islam after befriending exiled Chechen separatist leaders.

The report said there was "strong evidence" that he was seen within the FSB as "someone who had betrayed that organization."

He was buried in a London cemetery with Muslim rites in a lead-lined coffin to prevent radiation leakage.