Mr Zakayev, the exiled “Prime Minister” of the Chechen Republic, bitter enemy of President Putin and a close friend and confidante of Mr Berezovsky, has more reason than most to believe this.
After all, he was himself the target of an assassination plot in London, revealed by MI5 in April last year. It is the very frequency of these incidents that makes him doubt the theory that Mr Berezovsky took his own life. And certainly, with the death of Mr Berezovsky, President Putin has lost one of his most high-profile opponents.
In October 2006 Anna Politkovskaya, a campaigning journalist and vocal critic of the President, was shot dead at her Moscow home. A few weeks later, Alexander Litvinenko, a former KGB officer, died in London after his tea was spiked with radioactive polonium-210, on the suspected orders of the Kremlin.
Over the years, a series of senior Chechen separatists and opposition figures have been assassinated while in exile, one blown up by Russian military intelligence in his jeep in Qatar.
“First Anna is shot in Moscow, then Alexander is poisoned in London. My life is in constant danger. And now Boris Berezovsky is suddenly found dead? I’m not a proponent of conspiracy theories, but this raises many disturbing questions,” says Mr Zakayev, who knew all three victims well.
“Nobody among those who knew Berezovsky thinks it was suicide,” he goes on. “We all know the Russian secret service works on a world stage against Putin’s opponents and anyone who criticises his government. This death is part of a pattern.”
The body of the Russian oligarch was found by his bodyguard on the bathroom floor of his Ascot home. A ligature was round his neck and a piece of the same material was tied to the shower rail above his body.
The opening of the inquest into his death, held at Windsor Guildhall last Wednesday, heard that there had been no signs of a violent struggle.
The bare facts – pointing towards suicide – chimed with recent reports that Mr Berezovsky was depressed and had received treatment in Israel and at the Priory clinic.
His personal and business life had suffered several costly setbacks. Last year he lost a private action for £3 billion in damages against fellow oligarch Roman Abramovich, the owner of Chelsea FC, whom he had accused of blackmailing him into selling off business interests in the Sibneft oil giant at a fraction of their true worth.
It was the biggest private court case in British history and cost Mr Berezovsky millions. The previous year, he had been forced to hand over a reported £100 million in a divorce settlement to his former wife, Galina Besharova, 53, and only last January his former lover, Elena Gorbunova, began legal proceedings against him, claiming she too was owed millions.
The judge in that case described Mr Berezovsky as “a man under financial pressure”, an impression compounded when news emerged that the oligarch was attempting to sell a limited-edition Andy Warhol portrait of Lenin at Christie’s.
Some observers said Mr Berezovsky had recently begun to cut an increasingly isolated figure, worn down by recent defeats.
In Mr Berezovsky’s last interview, with a Russian reporter from Forbes magazine, he reflected that during his years in exile he had lost his bearings.
“I shouldn’t have left Russia,” he said the day before he died. “I lost the meaning… The meaning of life. I don’t want to engage in politics now. I don’t know what I should do now. I am 67 years old. And I don’t know what I should do.”
But such is the level of anxiety and distrust among opponents of the Russian president that the conclusion that Mr Berezovsky did indeed take his own life in a moment of despair has been greeted with scepticism and disbelief.
That was compounded when Thames Valley police admitted that nothing could be ruled out of their investigation until they had received the results of toxicology and histology tests on Mr Berezovsky. These are unlikely to be available for several weeks.
Mr Zakayev, a straight-backed, steel-haired former actor and friend of Vanessa Redgrave, insists there is nobody among those who truly knew his friend who believe the suicide theory.
Speaking to The Sunday Telegraph at the Hilton Hotel, in London’s Park Lane, a small badge of the Chechen Republic pinned to the lapel of his immaculate black suit, Mr Zakayev said: “I can assure you that Boris was not the kind of person who would harm himself.
“His friends and family can confirm that as well. He loved life and was not planning on leaving it any time soon.”
But the pair fell out and Mr Berezovsky and Mr Zakayev became political as well as personal friends, united by their hatred of President Putin.
Although Mr Berezovsky had recently been forced to reduce his security detail, some of whom were former members of the Foreign Legion, he had known his bodyguard for several years and, according to Mr Zakayev, trusted him completely.
Mr Berezovsky’s family have been left devastated by his death. In a brief statement issued last week, his 19-year-old daughter, Anastasia, described her father as an “extraordinary” man and spoke of the family’s huge sense of loss.
In the hours after his death, Mr Zakayev rang another of Mr Berezovsky’s daughters, Katya, who flew to France to comfort her grandmother, Berezovsky’s mother Anna, who is in her late eighties.
During the telephone conversation Katya, a Cambridge graduate, told Mr Zakayev: “This is a very hard time for us. We have been hit very hard by his death. But none of us believe he took his own life.
Katya was one of two children from Mr Berezovsky’s first wife, Nina. It fell to her sister, Elizaveta, an artist, to formally identify his body last week. Mr Berezovsky had two other children, Artem, 23, and Anastasia, from Galina, whom he married in 1991, and a further two, Arina and Gleb, from Miss Gorbunova.
He backed Yeltsin as president in 1996 and reaped the rewards when state-owned firms were put up for sale, buying the state airline Aeroflot and amassing a £3 billion fortune.
There are now moves by Russian prosecutors to confiscate Mr Berezovsky’s remaining wealth. Mr Zakayev cites this as another reason for the Kremlin to have an interest in his friend’s death.
Even before Mr Berezovsky’s death, those opponents of Mr Putin who fled the former Soviet Union to seek refuge in London had been under no illusion that their safety was guaranteed.
Before leaving – his next appointment is with Alexander Litvinenko’s widow, Marina, who had been assisted financially by Mr Berezovsky in her attempt to seek justice for her husband – Mr Zakayev points out that the British authorities believe there are now more Russian spies in London than during the Cold War.
“In 2006 the Russian parliament, under Putin’s initiative, passed a law allowing its secret services to liquidate 'enemies of the state’ inside Russia or even outside it,” he says, adding: “The suicide theory is very convenient for everyone, for both Russia and the British authorities. If it’s established that there has been yet another murder in the UK of someone who found protection here, it will discredit Britain and hurt her image.
“We must obviously await the outcome of the police investigation, but at the moment it doesn’t appear to add up. If a person hangs himself, then the cord and whatever it is attached to either breaks at the moment of the drop — in which case he survives – or they hold fast and the person dies. So how did Boris end up dead on the floor?
Several of Mr Berezovsky’s colleagues acknowledge that recent setbacks in his life had left him feeling “oppressed and tormented”, particularly his court defeat to Mr Abramovich. But Mr Zakayev denies his friend had lost either his ambition or the will to live. “Failures and disappointments, such as the ones he suffered recently, only made him stronger,” he said.