Alexander Litvinenko / Photo from www.telegraph.co.uk

The hit was "probably" carried out on the personal orders of the Russian president, Nico Hines, the London editor of The Daily Beast, wrote in an article titled "Litvinenko Was Killed 'for Calling Putin a Pedophile.'"

The allegation—that Putin had used his position as head of the Russian intelligence service to destroy video evidence of himself having sex with underage boys—was "the climax" of an increasingly bitter personal feud between Alexander Litvinenko and the Kremlin leader.

Sir Robert Owen, a retired High Court judge, found that this personal animosity, combined with Litvinenko's continued criticism of the Kremlin and the FSB, of which he was once a senior member, was the motive behind his brazen murder in a Mayfair hotel via a pot of green tea laced with the radioactive isotope polonium-210 in November 2006.

"The FSB operation to kill Mr. Litvinenko was probably approved by Mr. Patrushev, then head of the FSB, and also by President Putin," Owen told the Royal Courts of Justice on Thursday.

"There was undoubtedly a personal dimension to the antagonism between Mr. Litvinenko on the one hand and President Putin on the other," he wrote in his report. "Mr. Litvinenko made repeated highly personal attacks on President Putin culminating in the allegation of pedophilia in July 2006."

The claim was made in an article on the Chechen separatist website Chechenpress shortly after Putin was filmed lifting the T-shirt and kissing the stomach of a young boy at the Kremlin.

Litvinenko claimed this display of affection was the first public sign of a secret that had long been known by some within the KGB. He said Putin had been denied a place in the foreign intelligence division as a young recruit "because, shortly before his graduation, his bosses learned that Putin was a pedophile."

"Many years later, when Putin became the FSB director and was preparing for the presidency, he began to seek and destroy any compromising materials," Litvinenko wrote. "Among other things, Putin found videotapes in the FSB Internal Security directorate, which showed him making sex with some underage boys."

Thursday's announcement has been 10 years in the making. The British government rebuffed Marina Litvinenko's pleas for an inquiry into her husband's assassination for eight years because diplomats feared that London's improving relationship with Moscow would be severely damaged.

A secret letter written to Owen by the Home Secretary Theresa May in 2013 explained why no inquest had been allowed. "It is true that international relations have been a factor in the government's decision-making," she admitted.

The government would change its mind a year later, when Russia's conflict with Ukraine and the shooting down of passenger jet MH17 ended diplomatic niceties.

The explosive conclusion of the report will do little to calm relations between the nations, even though Owen stops short of saying he has conclusive proof that Putin ordered the hit directly.

The inquiry heard that two Russians, Andrei Lugovoy and Dmitry Kovtun, carried out the killing a few yards from the U.S. embassy in London by pouring polonium 210 into a pot of tea they offered to the former Russian spy.

Neither man still worked for the Russian state directly—Lugovoy had left the FSB and Kovtun was no longer employed by the Army. But Owen quoted an old Russian saying in his 300-page report: "There is no such thing as a former KGB man."