The inquiry into Alexander Litvinenko's murder has speculated the dissident could have been killed for claiming that Vladimir Putin had destroyed evidence of "paedophilia" involving the Russian president.
In a lengthy report Sir Robert Owen concluded Thursday that there is a "strong probability" that the FSB, successor to the Soviet spy agency the KGB, directed the killing, and the operation was "probably approved" by Putin.
Owen said that he is certain former KGB agent Litvinenko was given tea with a fatal dose of polonium-210, a radioactive isotope that is deadly if ingested even in tiny quantities, in London in November 2006.
The inquiry report includes a large section of the article, which says Putin destroyed videotapes of him having sex with "underage boys" when he was director of the FSB.
It references an alleged incident when he was president, when he lifted the t-shirt of a small boy to kiss him on the stomach, behaviour which baffled onlookers.
Owen called this article the "climax" of Litvinenko's attacks on Putin. The judge wrote: "It hardly needs saying that the allegations made by Mr Litvinenko against President Putin in this article were of the most serious nature.
"Could they have had any connection with his death?"
Home Secretary Theresa May said the Treasury has agreed to impose asset freezes against the two prime suspects in the murder, and that "senior representations" were being made to the authorities in Moscow, while the Russian ambassador was being summoned to the Foreign Office.
She told the Commons: "This was a blatant and unacceptable breach of the most fundamental tenets of international law and civilised behaviour. But we have to accept that this doesn't come as a surprise."
In summarising why the Russian state may have killed Litvinenko, Owen cited five reasons, including the "personal" hostility between Litvinenko and Putin, which "culminated" in the paedophile allegations.
"There was undoubtedly a personal dimension to the antagonism between Mr Litvinenko on the one hand and President Putin on the other," Owen writes.
"The history between the two men dated back to their (only) meeting in 1998, at a time when Mr Putin was the newly appointed head of the FSB and [oligarch Boris] Berezovsky and Mr Litvinenko still hoped that he might implement a programme of reform.
"In the years that followed, Mr Litvinenko made repeated highly personal attacks on President Putin, culminating in the allegation of paedophilia in July 2006."
The other four potential motives for the Russian state's involvement are Litvinenko's "betrayal" of the FSB, his work subsequent work with British intelligence, his association with fellow Putin critics like Berezovsky - who was granted political asylum in the UK and died of unknown circumstances in his Berkshire home in 2013 - and his allegations Putin colluded with organised crime.
Russia has already hit out at the Owen inquiry, calling it "political motivated".
An unnamed source told state news agency RIA: “Moscow will not accept the verdict of the British court in the Litvinenko case, London has violated the principle of presumption of innocence.”
Litvinenko implicated Putin in his poisoning from his deathbed.
His widow Marina Litvinenko said she "very pleased the words my husband spoke on his deathbed when he accused Mr Putin have been proven".
She added: "I'm also calling for the imposing of targeted economic sanctions and travel bans against named individuals ... including Mr Putin."
Labour shadow leader of the Commons Chris Bryant attacked the “murderous kleptomaniac regime in Russia”.
“It walks all over the weak. Putin has no respect for those who let him do what he wants,” he said, adding it should be made clear that “Russian murderers are not welcome in this country”.