Alexander Litvinenko: What were the motives for the former spy's murder?

President Vladimir Putin "probably" approved the assassination of former Russian spy Alexander Litvinenko in London in 2006, a report published on Thursday concluded.

But what were the possible motives for Mr Litvinenko's killing, whoever was responsible?

Here, we look at some of the motives put forward in Sir Robert Owen's report.

  • Betrayal of the FSB and criticism of Vladimir Putin

There was a perception within the FSB security service that Mr Litvinenko had betrayed the organisation, dating back to 1998 when he protested against orders he and his colleagues received.

After claiming asylum in the UK, he accused the FSB of corruption and mass murder.

While living in London, he gained a reputation as a campaigner and outspoken commentator, including writing two books critical of the FSB and alleging collusion between Vladimir Putin and organised criminals.

Mr Litvinenko was highly critical of Russian President Vladimir Putin. Credit: PA

  • Personal tension between Litvinenko and Putin

The report said there was "undoubtedly" personal antagonism between the pair, dating back to their only meeting, in 1998.

Mr Litvinenko went on to make repeated personal attacks on President Putin, including an allegation of paedophilia in July 2006.

  • Close ties with Boris Berezovsky

Mr Litvinenko developed a close relationship Boris Berezovsky, beginning in the mid-1990s.

Mr Berezovsky helped Vladimir Putin rise to power but fell out with him shortly after his election as president in March 2000 and left Russia at the end of 2000 to claim asylum in the UK, becoming a harsh critic of Mr Putin.

Mr Berezovsky was found dead at his home in March 2013, with an inquest recording an open verdict.

Russian business oligarch Boris Berezovsky fell out with Mr Putin. Credit: PA

  • Work for UK intelligence agencies

Since his death, there has been speculation Mr Litvinenko worked for British intelligence services, in particular MI6.

This has never been confirmed, but following his death one of the suspects in his killing, Andrei Lugovoi, said Mr Litvinenko had told him he worked with MI6 and tried to recruit him.

Whether this is true or not, the report says it "may not take much imagination to consider how the FSB would have reacted".

  • Work for the Mitrokhin Commission in Italy

Vasil Mitrokhin was a senior KGB archivist who defected to the UK in 1992, taking with him a large collection of KGB notes, which included details of historic operations around the world, such as in Italy.

In 2002, the Italian Parliament set up a commission to investigate matters in this archive, an inquiry Mr Litvinenko took part in and provided a large amount of information to, some of it highly sensitive.

Sir Robert Owen published his report on Thursday. Credit: PA

  • Work for the Spanish security services

Mr Litvinenko told Mr Lugovoi he was working with the Spanish secret service against the Russian mafia in Spain, evidence corroborated by Marina Litvinenko.

The report questions whether he was killed to punish him for assisting the Spanish authorities or to prevent him providing more help.