His professional life was lived in the secret world of the spy. After his defection to the West, he existed in the shadowy world of the exiled Russian oligarchs, who are so profoundly hated by Vladimir Putin. But he made sure that his death was front-page news, allegedly telling a friend: "The bastards got me, they won't get us all."

That was on Tuesday, the last day that he was able to communicate. He died on Thursday night, leaving a wife and teenage son and a statement dictated on his death bed in which he directly addressed the Russian president, saying: "You are as barbaric and ruthless as your most hostile critics have claimed." On Friday, Alexander Litvinenko's weeping father, Walter, branded the Russian government "a mortal danger to the worldf This regime got him"."

Scotland Yard's Counter Terrorism branch is calling it "an unexplained death". But there are plenty of people willing to explain it, including the late Alexander Litvinenko himself. "This is what it takes to prove one has been telling the truth, " he is reported to have said shortly before he lost consciousness. His friends in the Russian emigre community in Britain state baldly that he was murdered by FSB, the new name for the old KGB.

Deteriorating relationships THIS single death illuminates deteriorating relationships between the new Russia and the Western powers, now said to be at their lowest ebb since the Cold War. The flood of oil money into the new regime in the last three years has made the Kremlin bolder, just as its internal scandals and pressure on Ukraine and Georgia have made the West uneasy. In July of last year, Russian emigres suspected the worst when the Russian parliament, the Duma, passed a new law permitting the FSB to pursue and eliminate enemies of the state around the world. It is also worth noting that Alexander Litvinenko worked for the domestic wing of the old KGB, the FSB, and was charged with attacking corruption within Russia itself.

On Thursday night, Oleg Gordievsky, one of the most senior officers to defect from the old KGB, called Alexander Litvinenko "a hero of Russia and a hero of Britain". Gordievsky was quick to point out that Alexander Litvinenko had recently become a British citizen, and as such was a British victim of a murder by a foreign power, perpetrated on British territory.

The problem is that the murder weapon has yet to be found. Doctors at University College Hospital London have eliminated a range of poisons which sound like something from one of the more exotic James Bond plots, including Thallium in both its salt and radioactive forms. Now there is speculation that Litvinenko was killed by a cyto-toxic drug . . . a cell killer used to fight cancer. It is hoped that the organ analysis which will take place now may reveal what killed him. Police now say that they have found levels of radiation in the London sushi bar where he had eaten just before he fell ill.

Litvinenko had been in hospital for three weeks, during which time he went into kidney, liver and heart failure. His hair fell out. This ex-soldier, who prided himself on his physical fitness, looked like an old man. He was 44.

His Russian friends in Britain claim that Litvinenko was poisoned at a meeting at a London hotel with two old espionage colleagues from Russia.

Litvinenko had apparently met them because they had told him that they had information on the murder of investigative journalist Anna Politkovskaya, widely held to have been carried out by the FSB, which Litvinenko was investigating.

The world of exiled Russians These two Russians, Andrei Lugovoi and a man known simply as Vladimir, are said by Litvinenko's friends to have poisoned him. On Thursday, Andrei Lugovoi, in a highly unusual initiative, denied this from his office in Moscow.

"Someone is trying to set me up as the fall guy, " he told the Daily Telegraph. There was no food taken at the London meeting, he said, because he was in a rush. Interestingly, Lugovoi did not know Litvinenko during their time working for the KGB.

According to Lugovoi, they first met when he was working providing security for ORT, a television channel owned by the Russian oligarch Boris Berezovsky, for whom Litvinenko also worked.

Oleg Gordienvsky further stated that Vladimir Putin . . . a KGB veteran who once headed the FSB, in which Alexander Litvinenko had been a Lieutenant Colonel . . . must have known about the alleged poisoning. After the assassination of the anti-Putin journalist Politkovskaya . . . also the victim of a poisoning attempt . . . this makes for unwelcome international publicity for Vladimir Putin.

Relations between Moscow and London are said to be increasingly tense, despite furious denials from the FSB that it had anything to do with Litvinenko's death. The FSB has contemptuously implied that it would hardly bother to pursue a man of Litvinenko's rather low calibre.

But the opposition had already gathered, shortly after Litvinenko fell ill on 1 November. In Britain, he lived in the world of exiled Russians. He provided security for Boris Berezovsky, the mathemetician who became Russia's first billionaire under perestroika and who now lives as a fugitive in Surrey. Like many of the Russian oligarchs living in exile and wanted by the Putin government, Berezovsky is Jewish, and Russian anti-Semitism . . . always virulent . . . is surely a factor in the Kremlin's pursuit.

Leonid Nevzlin was the right-hand man of Mikhail Khodorkovsky, who once owned the Yukos oil company and is now in a Siberian labour camp.

Nevzlin is now wanted in Russia for murder. He lives in Israel, as does Vladimir Gusinsky, who fell foul of Putin when his independent television station became critical of the war in Chechnya . . . as Anna Politkovskaya had done. Moscow now accuses Vladimir Gusinsky of money laundering. He also lives in Israel.

Around Berezovsky in Surrey, gather a group of Russian exiles, furiously anti-Putin. The most notable of these is Akhmed Zakayev, once the culture minister in Chechnya and now effectively deputy leader of the Chechen underground government. Putin's government wanted Zakayev to stand trial for planning the siege of the Moscow Theatre. A British court refused to extradite him on the grounds that he could face torture in Moscow. Zakayev, a former actor, is provided with his personal security by Berezovsky. He lives next door to Litvinenko's home in Surrey.

Putin and the 'strong men' AT THE heart of all this is Putin's continuing relationship with the Russian security services. A spy for most of his professional life, Putin is thought to be most comfortable with, and trusting of, ex-KGB operatives. The Russians refer to them scathingly as the 'strong men', and Putin is said to rely on them more and more, appointing them to areas which have nothing to do with national or international security.

The men who make up the self-appointed international opposition to Putin, who is due to retire in 2008, were supporters of post-Soviet Russia, and made billions from it. Boris Berezovsky bankrolled Boris Yeltsin's second election and backed Putin in 2000. The late Litvinenko was a model soldier, joining the Russian army from school in 1980 and the KGB eight years later. He was promoted to the central office of the new FSB in 1991.

In 1998, at a sensational news conference at which his fellow FSB agents appeared in balaclavas, Alexander Litvinenko condemned his FSB bosses for ordering him to assasinate a Russian citizen. The Russian citizen was Boris Berezovsky.

The boss of the FSB in 1998 was Vladimir Putin.

Litvinenko was held in an FSB prison and twice cleared of charges against him. In 2000 he fled to the UK with his wife and son.

He then wrote a book, Blowing Up Russia, in which he alleged that the FSB had bombed the Moscow apartment buildings in which 300 people died in September 1999. Putin had blamed the outrage on Chechen terrorists, and he won the presidential election in March 2000.

Putin called Litvinenko's claim "delirious nonsense". However, 4,500 copies of Blowing Up Russia were confiscated by the FSB in Russia.

According to his friends, Litvinenko then embarked on a campaign of fearless criticism of the Putin regime, and of FSB corruption, most of which, they say, was published in the Chechen press.

The Kremlin strongly denies any involvement with Litvinenko's death. Critics of the oligarchs point out that there is a new extradition treaty between Russia and the UK in the pipeline, which would have profound implications for the oligarchs, all of whom are wanted for alleged crimes in Russia. This extradition treaty is presumably suspended, at the very least. It is left to the doctors and to the policemen to discover how Alexander Litvinenko died. In this labyrinth of intrigue, we may never know exactly why.

THE LEADING PLAYERS IN THE ESPIONAGE DRAMA VLADIMIR PUTIN: The Russian president was accused by Litvinenko of sanctioning his murder. The Kremlin has rejected the claims, but the death of the former agent will add to the perception that the FSB security service is running an assassination policy.

MARIO SCARAMELLA: The Italian academic and KGB expert met Litvinenko at a sushi restaurant on the day he fell ill. There is no suggestion that Scaramella was involved in the poisoning. He said he had met his friend to discuss a death threat aimed at them.

ANDREI LUGOVOY: Moscow-based businessman and former KGB bodyguard. Held a meeting with Litvinenko at a hotel on 1 November, when he had tea with Litvinenko and two other men. He said the meeting had been to discuss business and he had been in London to see a football match.

BORIS BEREZOVSKY: Exiled oligarch had become an ally of Litvinenko. A critic of Putin, he is thought to own the house in north London where Litvinenko was living and "nanced his book, which levelled corruption and murder allegations against the FSB and Kremlin.

AKHMED ZAKAYEV: Former actor and foreign minister of the Chechen government in exile was a visitor to Litvinenko's bedside. The two men were neighbours.

He accused the Kremlin of exporting "gangster politics" to London.

LORD TIM BELL: His PR firm includes Berezovsky among its clients. The company handled media calls about Litvinenko and arranged for the distribution of photographs taken of him in hospital.

ALEX GOLDFARB: The biochemist is director of a human rights group set up by Berezovsky in 2000. Goldfarb has put forward the allegation that the Kremlin is responsible for Litvinenko's death.

JOHN HENRY: Leading toxicologist.

Claimed that thallium was to blame for Litvinenko's condition. But the hospital said he had made his remarks without seeing test results. The professor said he has withdrawn from the case.