The finding will put pressure on the British government to take fresh measures against Russia, possibly including targeted sanctions and travel bans. It may also harm potential co-operation in military action against ISIS, and upcoming peace talks on the Syrian conflict.

Litvinenko died in November 2006 after a radioactive poison was slipped into his tea at a London hotel.

There was a "strong probability" that the two killers were under the direction of the FSB, Russia's security service.

"The FSB operation to kill Mr Litvinenko was probably approved by [then FSB head Nikolai] Patrushev and also by President Putin," Sir Robert Owen, who led the year-long inquiry, said.

The inquiry examined expert evidence and heard testimony from forensic scientists and family members, as well as secret evidence that was not disclosed in the public report - but believed to be from Western intelligence agencies.

Sir Robert said he was "sure" that Litvinenko was deliberately poisoned with the radioactive element polonium 210, which he ingested on November 1, 2006.

That afternoon Litvinenko had met two men for tea at the Pine Bar of the Millennium Hotel in Mayfair, London.

The men were Andrey Lugovoy and his associate Dmitri Kovtun – former Russian army officers. Lugovoy was a former KGB agent.

Forensic evidence showed the Pine Bar was "heavily contaminated" with polonium 210, the inquiry found.

"The highest readings were taken from the table where Mr Litvinenko was sitting and from the inside of one of the teapots. No comparable levels of contamination were found in any of the other places that Mr Litvinenko visited that day," the report said.

Sir Robert said he was sure that Lugovoy and Kovtun placed the polonium in the teapot at the Pine Bar. They had tried to kill him with the same poison at a meeting a few weeks earlier.

Kovtun and Lugovoy are wanted by British authorities on suspicion of the murder of Mr Litvinenko. A warrant has been issued for their arrest but Russia has not extradited them. Both have denied killing Mr Litvinenko.

Forensic scientists found "widespread radioactive contamination" at locations linked to Lugovoy, Kovtun and Mr Litvinenko in the weeks before he fell ill.

There were also high levels of radioactive contamination on the British Airways plane seats Kovtum and Lugovoy used when flying to Moscow two days after the murder, and in placed visited by Kovtun in Germany the week before he took met with Litvinenko.

The inquiry rejected a "chemical fingerprint" theory that definitively traced the polonium to a Russian factory in Sarov, though it "unquestionably" could have come from there.

Sir Robert also cast doubt on a claim by a 'Mr Potemkin' that the polonium came from an August 2006 shipment to the FSB in Moscow.

However, given the amount of polonium possessed and used by the assassins, it "strongly indicated" the involvement of a state, Sir Robert said.

"Ordinary criminals might have been expected to use a straightforward, less sophisticated means of killing… the polonium 210 used to kill Mr Litvinenko must have come from a reactor and such reactors are in general under state control."

The evidence in open court was strong circumstantial evidence of Russian state involvement, and the 'closed evidence' made it a strong probability that the FSB directed Lugovoy to poison Mr Litvinenko.

"There were powerful motives for organisations and individuals within the Russian state to take action against Mr Litvinenko, including killing him," Sir Robert said.

"Mr Litvinenko was … regarded as having betrayed the FSB, … was an associate of leading opponents of the Putin regime and he had repeatedly targeted President Putin himself with highly personal public criticism."

In one article, published the year he was killed, Mr Litvinenko claimed Mr Putin was a paedophile.

Evidence suggested Russia had previously killed a number of opponents of the Putin administration, through bombings and poison including radioactive poison.

Sir Robert said he was sure that Lugovoy and Kovtun were acting on behalf of others, probably the FSB.

Though they did not know the precise nature of the poison, they knew it was deadly, Sir Robert said.

During the inquiry Mr Putin awarded Lugovoy an honour for "services to the fatherland". He is now a member of the Russian parliament.

In a deathbed statement Mr Litvinenko accused Mr Putin of direct involvement in his murder.

The inquiry heard evidence from several of Mr Litvinenko's associates that the assassination could not have been done without Mr Putin's knowledge and approval.

"This is a KGB rule number one, cover your back," said one associate, Yuri Shvets.

An independent expert, Oxford University's Professor Robert Service, who studies Russian history, told the inquiry it was "inconceivable" that FSB head Mr Patrushev would not have had advance knowledge of the operation.

Professor Service said Mr Putin had "some oversight" of FSB operations, and Sir Robert concluded that Mr Patrushev probably would have told Mr Putin about an operation such as the murder of Mr Litvinenko, though it was at present "unprovable".

It was widely reported – and claimed by Mr Litvinenko's widow and associates – that Mr Litvinenko had worked for British intelligence service MI6 after his arrival in Britain.

Sir Robert said the British government had not provided any evidence on the question in the "open" part of the inquiry – but had not denied it, either.

Sir Robert said in any case it was more important whether the FSB believed he was working for British intelligence agencies, and "that is precisely what the FSB believed" according to Lugovoy.

Mr Litvinenko's former superior at the Russian secret service, Alexander Gusak, had agreed in an interview in 2007 that Litvinenko deserved to be executed because "when (he) defected abroad, he naturally handed over the undercover experts who had been its contacts".

Mr Litvinenko was born in December 1962 and was an officer in the KGB and then the FSB. He was dismissed in 1998 after making public allegations of illegal activity within the FSB.

He was granted asylum with his wife and son in Britain in 2001 and worked as a journalist and author and producing 'due diligence' reports on Russian individuals and companies.

He fell ill on the evening of November 1, 2006 and died on November 23.

Just before his death, experts realised his body was highly contaminated with radioactive polonium 201.

Sir Robert said he had made one recommendation as a result of his inquiry, but he could not reveal it publicly as it concerned the 'closed' evidence he had heard.

Mr Litvinenko's widow Marina said she was "very pleased" with the inquiry's findings.

She called for the British government to expel all Russian intelligence agents, "either FSB or other Russian agencies based in the London embassy".

She also called for immediate, targeted economic sanctions and travel bans against the people named in the report, including Mr Putin.

"It's unthinkable that the prime minister would do nothing in the face of (these) damning findings," she said.

Home Secretary Theresa May revealed she had also written to her counterparts in the EU, NATO and 'Five Eyes' countries - which includes Australia - drawing their attention to the report and the need to take steps "to prevent such a murder being committed on their streets".

She told parliament the report's finding that the Russian state was probably involved in the murder was "deeply disturbing" and a "blatant and unacceptable breach of fundamental international law".

She announced new asset freezes on the two alleged killers, saying Russia's "continued failure to ensure they are brought to justice is unacceptable".

The government had summoned the Russian ambassador to the Foreign Office to demand an account of the FSB's role in this case.

Lugovoy calls accusations 'absurd'

Andrei Lugovoy said the accusations against him were "absurd", the Interfax news agency quoted him as saying.

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 published on Thursday 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."

With Reuters

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