An outspoken critic of Putin, Berezovsky had sent him 'repentance' letter

Berezovsky had evidence of plot to topple President Putin, it is alleged

Boris Berezovsky was killed in his bathroom because he was preparing to give Vladimir Putin evidence of a plot involving leading oligarchs to topple the strongman in a coup, it was claimed today.

The exiled Russian tycoon was slain by Western secret services linked to the plan to overthrow the Kremlin leader.

The theory comes from Berezovsky's former long-time head of security Sergei Sokolov, who disputed the version of British police that the ex-billionaire took his own life, aged 67, in Berkshire in March 2013.

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'Convinced': Berezovsky's former head of security Sergei Sokolov says he knows his boss did not take his own life - and alleges he was killed by Western secret services over evidence of a plot to overthrow Putin

'I am convinced it was done by the Western secret services,' he said.

'If Berezovsky's death is a puzzle for you, it is a problem solved for me. Boris was killed simply because he possessed certain secret information and was going to make it public. This is not just my guess, I know it for sure.'

Sokolov, the head of a major Moscow private security agency, said his ex-boss was intent on returning to Russia, and had opened negotiations with Putin on allowing him back despite years of bitter opposition to the president.

'Boris forwarded Vladimir Putin a letter with repentance,' he said.

Deceased: Boris Berezovsky, left, was found dead at his ex-wife's home in Berkshire in March 2013. An outspoken critic of Putin, he is said to have sent the Kremlin leader a 'letter of repentance' and was planning his return to Russia

'It was a part of the big venture he had been planning. Berezovsky was living and looking forward to his return to Russia. Of course, he was hoping for indulgence.

'Boris was killed only because he possessed information which he was going to share with Putin.

'I am absolutely sure that Boris had video and audio evidence about certain oligarchs who offered to organise a coup in the country, promising him to be seen as being at the head of the attack.

'Berezovsky planned to come back to his motherland with this sensational material.

'You understand it now: those people he had gathered compromising materials for, considered him to be a serious threat.'

Berezovsky - who had a penchant for secretly recording conversations - had hidden his treasure trove, claimed Sokolov.

Allegations: The explosive theory was made by Sergei Sokolov, who claimed his former boss had gathered 'compromising material' as part of his plan to return to Russia

'Western secret services are still chasing it. Boris was not a predictable guy, who would keep precious information under the pillow in his house.'

He claimed that Berezovsky had inside information from MI6 about Russia.

'I knew that Boris was in close relations with MI6, Mossad and the CIA,' he claimed.

But 'when he became dangerous for their recruited residents such as Russian politicians and oligarchs, the Western secret services decided to get rid of him.'

He alleged: 'Berezovsky's murder was prepared for several months. It was a brilliant operation by the secret services.

'You decide: Berezovsky had a house .... round the clock security, CCTV cameras around the house. A fly would not make it.

'And on the day of his death all the security guards vanished, CCTV cameras were switched off for some reason, although they are placed every 100 metres.

'It is likely that not only MI6 took part in this operation but also CIA. There were people around Boris who were recruited by these secret services, including some of his guards.'

His personal guard and driver Avi Navam would later find his body in the locked bathroom.

He also disputed claims that Berezovsky was impoverished and depressed when he died having lost all his money, following a bitter £3 billion high court battle against Roman Abramovich.

Death: Berezovsky was found dead at his ex-wife's home, pictured, in Berkshire in March 2013.

'Let me assure you, Boris was not a poor man. There were rumours after his death that he did not have money for a plane ticket. This is just lie. At the moment of his death he had about $1 billion dollars in his bank accounts.

'One should agree, it is possible to enjoy your life with this money. Yes, he could not afford yachts like Abramovich but, gently speaking, he was not poor.'

He also had a string of young lovers, he claimed.

'I knew Boris very well. He was a man with solid mental health. He was full of adrenalin - it was obvious he was about to start a new venture.'

'Not a poor man': Berezovsky owed the taxman £46 million when he died, but Sokolov claimed he had about $1 billion in his bank account

Sokolov also claimed that in the 1990s, Berezovsky had ordered him to follow Boris Nemtsov, the Russian opposition leader killed in Moscow on 27 February.

He admitted to a Moscow newspaper that he had installed listening equipment in the politician's flat during the Yeltsin era.

'Berezovsky ordered us to follow Nemtsov,' he said.

'We tried to understand what he was interested in, and if he can tell the truth - we followed his movements plus installed recording equipment in his flat.

'Some part of compromising materials Berezovsky showed to Boris Yeltsin, something used for other purposes.'

'Every oligarch or official has a weak side,' he said.

'The best agent is the person who is next to you but you hardly notice him - either a secretary or a driver. It was easy to recruit women.'

He told Komsomolskaya Pravda newspaper: 'Nemtsov collected beautiful women.

'When young guys were going out with girl, he was studying, and then he got to the top of this life. And started to compensate it. This where his interest in young women was coming from.'

He claimed that Nemtsov could have been killed by a jealous husband.

'But first of all, Western secret services are the beneficiaries of Nemtsov's death.'

'From the point of view how it was planned, it is very much like CIA operation,' he claimed.

Despite this, Russian police have identified a Chechen link to the murder.

'It was a simple murder - go up the steps, shoot, jump into a car... and the killers, it does not matter who they are - Ukrainian or Chechen,' said Sokolov.

Negotiation: Berezovksy had opened talks with Putin about allowing him to come out of exile and return to Russia

A coroner last year recorded an open verdict at Berezovsky's inquest.

Mr Berezovsky, 67, was found with a ligature around his neck.

Berkshire coroner Peter Bedford said that 'contradictory' evidence meant he could not prove beyond all reasonable doubt that the businessman either took his own life or was unlawfully killed.

Sokolov was head of the Atoll security agency, and worked with Berezovsky from 1994 to 1999.

The security specialist has changed his mind on the tycoon's death which he originally saw as suicide.

Soon after his body was found, Sokolov said: 'Berezovsky was not a threat to anybody and nobody really needed him.