Russian President Vladimir Putin "probably" approved the assassination of ex-KGB agent turned dissident Alexander Litvinenko in London, a British inquiry has found.

In what many analysts saw as a sensational verdict, Judge Robert Owen said on Thursday that it was likely the Russian leader signed off the killing of the former spy in 2006 after a long-running feud.

Owen's 300-page report said Andrei Lugovoi and Dmitri Kovtun were probably acting under the direction of Moscow's FSB intelligence service, successor of the KGB, when they poisoned the 43-year-old with radioactive polonium 210 at London's Millennium Hotel.

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Russia's foreign ministry was swift to respond, dismissing the inquiry as "biased" and "opaque", according to the official RIA news agency.

"Moscow had no expectation that London's report on Litvinenko would all of a sudden become impartial," Maria Zakharova, a spokeswoman, said.

The Interfax news agency quoted the accused Lugovoi, who is now a politician, as saying: "This is a poor attempt from London to use a skeleton in the closet to the advantage of their political position."

Litvinenko, who lived in exile in Britain, died in November 2006 three weeks after drinking green tea laced with poison at the plush hotel.

British police had accused Kovtun and Lugovoi, the two Russians he met for tea, of carrying out the killing. Both denied involvement, and Moscow refused to extradite them.

Singling out Nikolai Patruskev, the head of the FSB at the time, alongside Putin, Owen wrote: "Taking full account of all the evidence and analysis available to me I find that the FSB operation to kill Litvinenko was probably approved by Mr Patrushev and also by President Putin."

British Interior Minister Theresa May said on Thursday the UK government would freeze the assets of Lugovoi and Kovtun and would summon the Russian ambassador to London to express its "profound displeasure".

May also told politicians that the conclusion that the Russian state is probably involved in the murder of Litvinenko was "deeply disturbing". She described it as a "blatant and unacceptable breach of international law and civilised behaviour".

Skin turned yellow

From his deathbed, Litvinenko had told detectives that he believed Putin directly ordered his killing. The Kremlin dismissed the claim as ridiculous at the time and has always vehemently denied any involvement.

The inquiry heard from 62 witnesses over six months of public hearings and - behind closed doors - saw secret intelligence evidence about Litvinenko and his links to UK spy agencies.

Litvinenko's widow Marina told the inquiry that her husband was a loyal intelligence agent who grew disillusioned with Russia's 1990s war in Chechnya and by what he saw as corruption within the FSB security service, the successor to the KGB.

Speaking outside the High Court after the verdict, she said 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".

When he became violently ill, Litvinenko's doctors diagnosed him with a stomach infection. But as his condition worsened, his white blood cell count plummeted, making him susceptible to infection.

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"His skin had turned yellow, indicating liver dysfunction, and he was tested for the two most likely causes, hepatitis and AIDS, but neither was the case," John Emsley wrote in Molecules of Murder, a crime book that includes a chapter on polonium poisoning.

"Then his hair began to fall out."

A diplomatic low

Doctors eventually decided that he was suffering from radiation poisoning, and further tests identified polonium as the culprit.

Litvinenko's body was so radioactive that the autopsy was conducted by medics in protective clothing and ventilation hoods. A lawyer for the police said that the killing may have exposed hundreds or even thousands of Londoners to radioactive contamination.

The former secret agent's death marked a post-Cold War low point in Anglo-Russian relations, and ties have never fully recovered. They were marred further in recent years by disputes over the conflict in Crimea and by Russia's support for Syrian President Bashar al-Assad, who the UK opposes.

British newspapers said UK Prime Minister David Cameron would chair a meeting of security chiefs before publication of the report to consider what, if any, action Britain should take.

READ MORE: Poisoned spy inquiry reignites British-Russian tensions

Some analysts believe, though, that it may be in the interests of both Britain and Russia to limit any fallout.

Both are involved in air strikes against the Islamic State of Iraq and the Levant (ISIL). British diplomats believe Russia is key to ending that country's civil war, while Russia would like to see an end to sanctions imposed on it by the West over Crimea.

The Soviet-era KGB did not hesitate to kill its enemies on foreign soil, sometimes with obscure poisons. Bulgarian dissident Georgi Markov died after he was stabbed with a ricin-tipped umbrella on London's Waterloo Bridge in 1978.