Flanked by fellow agents in balaclavas, he accused the FSB, the restyled KGB, of ordering him to assassinate the Russian oligarch Boris Berezovsky in November 1998. The head of the spy agency then was Vladimir Putin.

Mr Litvinenko, now 44, was a lieutenant colonel in the FSB’s elite unit fighting terrorism and organised crime. He claimed that many of his inquiries implicated top FSB officials in extortion and contract killings, a fact that he says he brought personally to Mr Putin’s attention only to be suspended shortly afterwards.

He was arrested for alleged corruption and placed in the FSB’s infamous Lefortovo prison. A court cleared him in November 1999, but he was rearrested and tried on fresh charges, only to be cleared a second time.

Mr Litvinenko fled Russia as a third case was being prepared. He arrived in Britain in October 2000 and has lived in London with his wife, Marina, and son, Anatoly, ever since.

Mr Litvinenko, who became a British citizen last month, alleged in a book published in 2002 that the FSB was behind bomb attacks in apartment buildings in Russia that killed 300 people in September 1999.

Mr Putin, then Prime Minister, had blamed the bombings on Chechen terrorists and, two weeks later, ordered troops into Chechnya to crush a separatist movement. Patriotic fervour stirred by the war swept Mr Putin into the Kremlin in the presidential election of March 2000.

He dismissed his former subordinate’s claims as “delirious nonsense”. Nevertheless, the FSB confiscated 4,500 copies of the book, Blowing Up Russia, for allegedly disclosing state secrets.

Before his decision to denounce his superiors, Mr Litvinenko had risen through the ranks of the Soviet military, which he joined from school in 1980. He joined the KGB’s counter-intelligence section in 1988 and was promoted to the central staff of the FSB in 1991.

He was convicted in absentia of abuse of office by a Moscow court in May 2002. He has become a close associate of Mr Berezovsky, who also fled to London in 2000. The billionaire businessman, who was given asylum in 2003, had supported Mr Putin in his rise to power but is now one of the President’s fiercest critics.

Times Recommends

• Obama: the world can expect better of America
• Gulp! Mystery killer threatens French oyster
• Secret of Northn Lights key to space safety