Alexander Litvinenko may have been killed after a deal that went wrong with associates involved in the ruthless world of Russian business.

According to security sources, investigators are looking at the former spy’s dealings with Russian businessmen involved in the lucrative energy sector and the shadowy world of private security. “We are looking at a very long list of Mr Litvinenko’s friends and foes since he has been in London,” one source said.

The list includes exotic figures ranging from billionaire businessmen, former Kremlin spies and KGB agents to underworld bosses.

In the six years that he was in Britain, Litvinenko appeared to have acquired a formidable collection of friends and enemies. Although he described himself as a journalist, Litvinenko tried unsuccessfully to muscle in on several lucrative business deals with Russians.

On the day that he fell ill he was attempting to broker a gas and oil exploration deal involving a British conglomerate that he claimed to represent. He was envious of the money that many of his former colleagues were making.

He also had talks about providing trained personal protection guards recruited from Russia, and claimed to represent a number of British interests wanting bilateral deals with Russian investors.

Police will look at investigations that his friends say he claimed to be involved in at the time of his death, including smuggling rings for nuclear material and prostitutes.

People connected to this world are frequently murdered on the streets of Russia’s cities, but until now the practice has not spread to London’s large Russian expatriate community.

The latest line of inquiry will confuse further an already complex investigation with a cast of characters that already includes President Putin, his nemesis Boris Berezovsky, the Russian oligarch exiled in London, rogue FSB death squads and the Chechen mafia.

Even now counter-terrorist detectives have pointedly not used the word “murder”, preferring “suspicious death”.

Much of the latest focus of attention has been on Andrei Lugovoy, a former Russian intelligence officer, who met Litvinenko on the day he was poisoned.

There is no evidence to suggest that he had anything to do with Litvinenko’s death, but suspicions about him deepened this week after the suspected poisoning of Yegor Gaidar, the former Russian Prime Minister and Putin critic.

Mr Gaidar, 50, was recovering in a Moscow hospital from a mystery illness that he contracted on a visit to the Irish Republic last week.

Mr Lugovoy was Mr Gaidar’s chief bodyguard in the 1990s. Although the two have not met for four years, Mr Lugovoy emerged as the one man linking the two cases.

The focus on his activities has not distracted attention from the Kremlin. Mr Putin’s many critics have accused the former KGB chief of launching a campaign to silence, intimidate and eliminate his critics and opponents.

Litvinenko became one of Mr Putin’s most outspoken critics after writing a book accusing the Russian leader of orchestrating a series of apartment-block explosions that were blamed on Chechen terrorists. But Western officials doubt that Mr Putin would have ordered the assassination.

The Kremlin has pointed the finger of suspicion firmly at Mr Berezovsky. Russian officials maintain that the oligarch has gained most from seeing Mr Putin’s reputation tarnished by the death.

Although Mr Berezovsky was an ally of Litvinenko, there are also suggestions that the two men could have fallen out. On the day he died, Litvinenko visited the oligarch’s Mayfair offices, which have since been sealed because they contain traces of polonium-210.

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