Owen's 300-page report said Andrei Lugovoi and Dmitri Kovtun — the two main suspects in the killing — were probably acting under the direction of Russia's intelligence service, the FSB, when they poisoned Litvinenko, 43, at London's Millennium Hotel. Owen added that it was likely that Putin signed off the killing of the former spy after a long-running feud.

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," said Maria Zakharova, a ministry spokeswoman.

The Interfax news agency quoted 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."

The British government said it would summon Russia's ambassador. "The conclusion that the Russian state was probably involved in the murder of Mr. Litvinenko is deeply disturbing," Interior Minister Theresa May told Parliament. "This was a blatant and unacceptable breach of the most fundamental tenets of international law and of civilized behavior."

Litvinenko, who was living in exile in Britain at the time, died three weeks after drinking green tea laced with polonium at the plush hotel. British police 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.

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 [then–FSB chief Nikolai] Patrushev and also by President Putin."

From his deathbed, Litvinenko told detectives he believed Putin directly ordered his killing. The Kremlin dismissed that 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 U.K. spy agencies.

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

Speaking outside the high court after the report was released, 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, Alexander Litvinenko's doctors diagnosed him with a stomach infection. As his condition worsened, his white blood cell count plummeted, making him susceptible to infection.

"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."

Doctors eventually concluded 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 examiners in protective clothing and ventilation hoods. A lawyer for the police said the killing may have exposed hundreds or even thousands of Londoners to radioactive contamination.

The former secret agent's killing led to 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 Russia's annexation of Crimea and by Moscow's support for Syrian President Bashar al-Assad, whom the U.K. opposes.

British newspapers said U.K. 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.

Some analysts believe, though, that it may be in the interests of both Britain and Russia to limit any fallout. Both are involved in airstrikes against the Islamic State in Iraq and the Levant. British diplomats believe Russia is key to ending that country's civil war, and 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.

Al Jazeera and wire services