Russian President Vladimir Putin "probably" gave the go-ahead for the murder with poison of former FSB intelligence service officer Alexander Litvinenko in 2006, a public inquiry conducted in the UK said on January 21 about the causes of death. 

Speaking at the High Court in London as a report of the findings was released, inquiry chairman Sir Robert Owen said Litvinenko was targetted in a special operation by the Federal Security Service, the main successor organisation to the Soviet KGB. The assassination was thought to have been ordered by former FSB director Nikolai Patrushev with a nod from the Russian leader.

"The FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev and also by President Putin," Owen said. His murder was in part due to personal "antagonism" with the president, he added.

Litvinenko died in London on November 23, 2006, days after drinking tea laced with the radioactive substance polonium-210.

Other possible motives include his revelations about the work of the FSB, his criticism of the Russian security services and relations with other critics of Russia's political regime, such as exiled oligarch Boris Berezovsky, who was found dead at his home in Britain in 2013.

The 43-year-old lieutenant-colonel fled to Britain after accusing the FSB of plotting to assassinate Berezovsky and went on to become a fierce Kremlin critic, author and consultant for the British intelligence services.

British Home Secretary Theresa May said his murder was a "blatant and unacceptable" breach of international law, while the Russian Foreign Ministry said it had yet to study the report in full but regarded it as "politicised".

"We regret that the purely criminal case was politicised and overshadowed the general atmosphere of bilateral relations," ministry spokeswoman Maria Zakharova said.

Russian citizens Andrei Lugovoi, the prime suspect and now a member of the Russian parliament, and Dmitry Kovtun are to be held responsible for the murder, Lord Owen said: "I am sure that Mr Lugovoi or Mr Kovtun placed the polonium-210 into the teapot with the intention of poisoning Mr Litvinenko."

He believes both men knew they were using a deadly poison, and not a sleeping draught or truth drug, although they might not have known exactly what that substance was, the report states. Both deny any involvement in Litvinenko's death.

"As we expected, the sensation did not happen!" State Duma deputy Lugovoi said, dismissing the accusations against him. "The results of the investigation announced today once again confirm London's anti-Russian stance, narrow-mindedness and unwillingness of the British to establish the true cause of Litvinenko's death," Interfax quoted him as saying.

According to Lugovoi, the "polonium scandal" has become a convenient way for London to achieve its political goals. Kovtun declined to comment, saying he needed more information about the hearing's findings.

In a statement to the House of Commons, the British Home Secretary said the UK would now impose asset freezes on both men and that international arrest warrants against the pair also remained in place. Prime Minister David Cameron would also raise the findings with Putin at "the next available opportunity", May added.

Russia authorities have refused to extradite the two suspects, and the country already has a constitutional ban on extradition altogether. According to two Scotland Yard detectives who travelled to Moscow to seek evidence, the Russian side continually hampered the investigation.

Foreign Ministry spokesperson Zakharova said that despite its title as a "public inquiry", the investigation, "was not transparent for Russia or for the public, since certain materials were considered in a closed court under the pretext of secrecy", Interfax reported.

Meanwhile, Litvinenko's widow Marina welcomed the report and called for further sanctions to be imposed on Russia and a travel ban on Putin.