LONDON (REUTERS/BLOOMBERG/AFP) - Russian President Vladimir Putin probably approved a Russian intelligence operation to murder former KGB agent Alexander Litvinenko, a judge led-British inquiry into the 2006 killing in London concluded.

There was personal antagonism between the men, and Mr Putin and members of his administration had motives for killing him, the inquiry published in London on Thursday (Jan 21) said.

Mr Litvinenko, 43, an outspoken critic of Mr Putin who fled Russia six years before his murder, died three weeks after drinking green tea laced with the rare radioactive isotope polonium-210 at a London hotel.

He blamed his death on Mr Putin in an impassioned statement from his hospital bed.

The Kremlin has always denied any involvement and dismissed the latest accusations as a "joke".

"Maybe this is a joke," Putin's spokesman Dmitry Peskov told reporters.

"More likely it can be attributed to fine British humour - the fact that an open public inquiry is based on the classified data of special services, unnamed special services, and that the verdict which has been made on the basis of this flimsy data has been made public with the copious use of the words 'probably' and 'likely'."

He also said that such a "quasi-inquiry" could only "add more poison to the atmosphere of our bilateral ties."

British Prime Minister David Cameron later said Litvinenko was murdered in a "state-sponsored action".

"What happened was absolutely appalling and this report confirms what we've always believed, and what the last Labour government believed at the time of this dreadful murder, which is it was state sponsored action," he said in a clip to British television from Davos.

The poisoners were former KGB bodyguard-turned-lawmaker Andrei Lugovoy and fellow Russian Dmitry Kovtun, the report by Senior Judge Robert Owen said.

Both men have denied involvement and Russia has refused to extradite them. Mr Lugovoy was quoted by the Interfax news agency as saying the accusation was absurd.

Mr Lugovoy, who represents the ultra-nationalist Liberal Democratic Party of Russia in the Russian Parliament, called the British inquiry "a pathetic attempt by London to use a skeleton in the closet for the sake of its political ambitions".

He said the findings of the inquiry continued Britain's "anti-Russian hysteria", which he said began after "the events in Ukraine in 2014".

"The accusations brought against me are absurd," he said. "As we expected, there was no sensation. The results of the inquiry published today are yet more proof of London's anti-Russian stance, its blinkered thinking and ... unwillingness to establish the true cause of Litvinenko's death."

Mr Litvinenko's widow called for sanctions against Russia and a travel ban on Mr Putin.

Mrs Marina Litvinenko called "for the imposition of targeted economic sanctions and travel bans against named individuals including Mr Patrushev and Mr Putin".

She also issued a direct message to British Prime Minister David Cameron, "calling immediately for (the) expulsion from the UK of all Russian intelligence operatives... based at the London embassy".

The widow added she was "very pleased that the words my husband spoke on his deathbed when he accused Mr Putin have been proved by an English court".

A spokesman for Mr Cameron said Britain is considering taking further action against Russia. "The conclusion that the murder was authorized at the highest levels of the Russian state is extremely disturbing," the spokesman told reporters. "It is not the way for any state, let alone a permanent member of the United Nations Security Council, to behave." "In light of the inquiry's findings we are considering what further actions we should take," she said.

British Interior Minister Theresa May said the government would summon Russia's ambassador to London to express its displeasure at Moscow's failure to cooperate with the investigation into Mr Litvinenko's murder.

Ms May also told Parliament that asset freezes would be put in place relating to the case.

The Russian Foreign Ministry on Thursday dismissed the outcome of the inquiry as biased and opaque, saying London's handling of the case had clouded bilateral ties.

Ms Maria Zakharova, a spokesman for the ministry, accused Britain of politicising the matter. "We regret that what was a purely criminal case was politicised and has clouded the general atmosphere of our bilateral ties," she told reporters. "The process ... was not transparent for the Russian side or for society because of the way materials were examined behind closed doors under the pretext that they were secret."

She said the final outcome was therefore the result of a"politically motivated and extremely opaque process".

The report says forensic evidence indicates that Mr Lugovoi and Mr Kovtun deliberately poisoned Mr Litvinenko with the radioactive tea, probably under the direction of the FSB security service.

"Taking full account of all the evidence and analysis available to me, I find that the FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev and also by President Putin," the inquiry said.

Mr Nikolai Patrushev was formerly head of the FSB, the security agency that succeeded the KGB.

"Members of the Putin administration, including the president himself and the FSB, had motives for taking action against Mr Litvinenko, including killing him," Judge Owen said.

"President Putin's conduct towards Mr Lugovoi suggests a level of approval for the killing."

Since the inquiry began in January 2015, Anglo-Russian diplomatic relations have sunk to their lowest point since the Cold War and the report may further strain the relationship.

The investigation pored over closed-circuit television footage that retraced Mr Litvinenko's final weeks, and examined his complicated relationships with British spy agencies and oligarchs.