He was the former KGB agent who claimed to know about the plot to kill Alexander Litvinenko and the dirty secrets behind the rise of President Putin.

Mikhail Trepashkin was freed yesterday after serving four years in prison, only two days before parliamentary elections that have become a referendum on Mr Putin’s popularity. He said that he felt he had been released because the Kremlin no longer regarded him as a threat.

Mr Trepashkin told The Times: “They think that Putin’s rating is so high now that I am no longer important for them.”

Mr Putin wants victory for his United Russia party in tomorrow’s elections so that he has the “moral authority” to hold the Government to account after he steps down as President in March. Many analysts believe that a victory would pave the way for Mr Putin to return to the Kremlin by “popular demand” after a short interregnum involving a loyal puppet, to get around the constitutional ban on a third consecutive term.

Opposition parties have complained of bias on state-controlled television and evidence has emerged of threats by government bodies to public sector workers to back United Russia or lose their jobs. Garry Kasparov, the chess champion who was jailed for five days this week for an antiPutin protest, said: “The volume of falsification, violations and the volume of suppression of opponents already makes the current regime illegitimate.”

When he emerged from prison in Nizhny Tagil, 900 miles (1,450km) east of Moscow, Mr Trepashkin vowed to renew his inquiry into allegations that the Federal Security Service (FSB), the KGB’s successor, bombed apartment buildings that killed 300 in September 1999. Mr Trepashkin was arrested in 2003, days before he was due to give evidence that would have implicated the FSB in a trial related to the bombings. He was later convicted of disclosing state secrets.

Mr Putin was promoted from director of the FSB to Prime Minister soon before the bombings. He blamed them on Chechen terrorists and launched a war that swept him to the presidency on a wave of patriotic fervour in March 2000.

“I can’t say that Putin himself ordered it because I don’t have that information. But I think that it was somebody from his team and that he knew all about this,” Mr Trepashkin said.

In a startling twist, Mr Trepashkin also said that his friend Mr Litvinenko, who was posioned with radioactive polonium210 in London last year, may not have been killed knowingly by Britain’s prime suspect, Andrei Lugovoy. He said that the FSB may have used Mr Lugovoy as an unwitting assassin in a 2002 plot that originally had him in that role. Mr Trepashkin was prevented from speaking to detectives at Scotland Yard from prison last December.

Mr Trepashkin said that a former FSB colonel had told him in 2002 that a group had been established to kill Mr Litvinenko and others linked with his book that accuses the FSB of carrying out the apartment bombings. The officer had urged him to travel to London to meet Mr Litvinenko so that FSB agents could establish where he lived, but he refused.

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