UPDATE 17:22 IST: British government says it will freeze assets of two Russians accused of killing Alexander Litvinenko.
UK Home Secretary Theresa May called the murder a "blatant and unacceptable breach of the most fundamental tenants of international law and civil behaviour," according to The Guardian.
UPDATE 15:51 IST: Russia has rejected the verdict of the inquiry calling it politically motivated.
"We regret that the purely criminal case has been politicised and has marred the entire atmosphere of bilateral relations, Maria Zakharova, spokesperson for Russia's foreign ministry, said on Thursday, according to TASS.
"The reason that London's inquiry was not transparent is clear taking into consideration that the materials were considered under pretext of secrecy," she said.
The official said that Russia will give a "full assessment" after studying the report.
UPDATE 15:37 IST: Former KGB bodyguard Andrei Lugovoi, who has been accused by the British inquiry of poisoning Litvinenko's tea with polonium, has called the charges "absurd", Interfax reported.
The Russian agency also cited a source saying that Russia will not turn over the two accused to the United Kingdom.
The report of a public inquiry into the 2006 murder of former Russian spy Alexander Litvinenko in London was released on Thursday, and claims Russian President Vladimir Putin "probably approved" the poisoning.
Litvinenko died days after drinking tea laced with the lethal radioactive polonium-210 while meeting with former Russian agents Andrei Lugovoi and Dmitri Kovtun in London.
The inquiry has found Lugovoi and Kovtun murdered Litvinenko "acting on behalf of others", according to the report. The report also highlighted that the two had attempted to poison Litvinenko about 15 days before they succeeded in poisoning his tea on 1 November 2006.
"I am sure that Lugovoy and Kovtun knew that they were using a deadly poison, and that they intended to kill Litvinenko. I do not believe, however, that they knew precisely what the chemical that they were handling was, or the nature of all its properties," judge Robert Owen said in the report.
The report also named Russian spy chief Nikolai Patrushev as having given an approval for the murder.
"The FSB operation to kill Litvinenko was probably approved by Patrushev and also by President Putin," the British judge said.
Litvinenko had moved to London in 2000 after fleeing Russia and had taken to writing against Putin and the Kremlin. He had reportedly found evidence of Putin and Patrushev's collusion with an organised crime group that smuggled heroin, when he was posted with the erstwhile Soviet Union's Committee for State Security (KGB).
A public inquiry into his death had begun in January last year.
There was outrage last year when Putin had awarded Lugovoi with a medal for "services to the motherland". The Russian President had rejected a request by the UK to extradite Lugovoi in 2007.
Litvinenko's widow said following the verdict that "the words my husband spoke on his deathbed when he accused Mr Putin have been proved by an English court", BBC reported.