Boris Berezovsky, a multi-millionaire businessman and a former Kremlin adviser and deputy of the Duma, the Russian parliament, said that the Federal Security Services (FSB) carried out the attacks with Mr Putin’s knowledge.

At a press conference in the Royal United Services Institute in Whitehall, Mr Berezovsky said that he had reached the conclusion after a year-long private investigation. His views were supported by liberal Russian MPs, explosives experts, documentary film-makers and a bomb survivor.

“I am not saying that he (Putin) gave the orders, but that he knew about it,” Mr Berezovsky said. “He knew definitely that such things were taking place. This is a fact.”

The allegations brought an angry denial from Moscow, which accused Mr Berezovsky of working in league with Chechen rebels, who were blamed by the Kremlin for carrying out the bombings.

More than 300 people were killed by two explosions in Moscow and one in the city of Volgodonsk in September 1999. No one has been convicted for the attacks, but suspicions of an official cover-up were raised after police in the city of Ryazan found and defused a further bomb.

The FSB, the successor to the KGB, said that it had planted the device in the basement of a block of flats to test local civil defence and that it contained harmless bags of sugar.

Whatever the truth, the blasts provided the pretext for the second Russian invasion of Chechnya. Mr Putin, then Prime Minister, saw his popularity rise as a result of the conflict and he was elected President the following year.

The Kremlin insisted yesterday that the allegations were the work of wild conspiracy theorists backed by Mr Berezovsky, who was forced into exile last year after falling out with the Putin administration.

Pavel Barkovsky, the head of the special investigations department at the Russian Prosecutor-General’s Office, accused the businessman of acting as the “financial sponsor of international terrorists”. He said: “We have information that money was transferred both under the guise of ransoms for people abducted by Chechen rebels and financing the restoration of the Chechen economy.”

Mr Berezovsky, who once controlled television channels, car-making, oil exports and airlines, was a key player in deals to free hostages held by the Chechens in the mid-1990s. As deputy director of the Security Council under President Yeltsin, he helped to broker deals to release Russian paramilitaries, soldiers and journalists.

Mr Berezovsky’s alleged proof of the Russian special services’ involvement in the bombings was met with widespread cynicism in Moscow yesterday. Politicians accused him of pure self-promotion.

“When Berezovsky was in power, he campaigned for war. When he was considered persona non grata he moved abroad and made a documentary,” Gennadi Zyuganov, the Communist Party leader, said.

Mr Berezovsky is so reviled in Russia that any claims are unlikely to be taken seriously. Russians view him as one of the “new rich” who profited from the chaos of the early post-communist days to make fortunes with scant regard to the law. The reinvention of a Kremlin kingmaker as a defender of human rights is, say Russian observers, highly unconvincing.