Five theories Possible motives include accusing Putin of paedophilia, links to oligarchs or his work for MI6

Alexander Litvinenko accusing Vladimir Putin of being a paedophile was one possible reason for his assassination, the public inquiry found.

Chairman Sir Robert Owen said “personal antagonism” between the pair, which dated back to 1998, was among five “powerful motives” for the murder.

That tension “culminated in (an) allegation of paedophilia in 2006” – just four months before he was killed.

He claimed Mr Putin had destroyed videos which showed him “making sex with some underage boys”.

Mr Litvinenko said: “The explanation may be found if we look carefully at the so-called ‘blank spots’ in Putin’s biography.”

He said that after graduating from the KGB college he was not accepted into the foreign intelligence service because “shortly before his graduation, his bosses learned that Putin was a pedophile [sic].”

Instead of making a fuss, officials at the Andropov Institute, the KGB college, decided to avoid sending him abroad. “Many years later, when Putin became the FSB director and was preparing for the presidency, he began to seek and destroy any compromising materials collected against him by the secret services over earlier years... among other things, Putin found video tapes which showed him making sex with some underage boys.”

Other possible motives included a belief that the former spy had betrayed the FSB – the successor to the KGB – through public disclosures about its work including a plot to murder Boris Berezovsky, the exiled oligarch, and books linking it to an apartment bombing in 1999 and collusion with organised crime. The FSB also had “information that Mr Litvinenko was working for British intelligence”. This was true and he was helping MI6 and other foreign intelligence agencies, being paid £2,000 a month.

The report said: “It may not take much imagination to consider how the FSB would have reacted to a report that one of its own former officers was working with British intelligence.”

Mr Litvinenko was also a prominent associate of leading opponents of Mr Putin and his regime, including Mr Berezovsky.

Finally, the report said, Mr Litvinenko’s claims about the FSB were “areas of particular sensitivity to the Putin administration”.

The report also dismissed theories that he had killed himself or had been murdered on the orders of Mr Berezovsky, UK agencies or Russian crime lords.