The theory: With presidential elections due in 2008, the Kremlin is eliminating any obstacles to a smooth transfer of power to Putin's chosen successor.

The case for: As a former FSB agent and author of a book accusing the agency of blowing up apartment buildings in Russia, killing 300 people, in 1999. The attacks were blamed on Chechen terrorists, provoking the second Chechen war, which propelled Putin into the presidency. Litvinenko knew secrets that would have discredited the Kremlin at a critical moment in the elections.

The case against: Any secrets that were held by Litvinenko would be at least a decade old by the time of the elections, and his most incendiary allegation has already been published to little effect. The Russian public generally views Litvinenko as a traitor — that is, if they have heard of him at all — and are therefore ill-disposed to believe him.

The hand of Berezovsky

The theory: The exiled oligarch is locked in mortal combat with Vladimir Putin and will stop at nothing destroy the President's reputation in the West.

The case for: Berezovksy initially backed Putin's rise to power but the former KGB boss ruthlessly dismantled his business empire in Russia in return. Forced to flee abroad, Berezovsky plots revenge. Komsomolskaya Pravda, the largest-selling Moscow daily, said that Berezovsky had most to gain by the deaths of his colleagues. The journalist Anna Politkovskaya was killed just before Putin's official visit to Germany and now Litvinenko has died on the eve of the EU-Russia summit.

The case against: Berezovsky has been vocal in accusing the Kremlin of poisoning Litvinenko, but it is a big stretch to believe that he would stoop to ordering assassinations in Britain — even by the Machiavellian standards of contemporary Russian politics. Besides, he would not risk his status as a political refugee, the only thing preventing his extradition to Russia and certain jail.

The Chechen connection

The theory: Litvinenko became a victim of a struggle for control of Chechnya because of his links in London with the exiled rebel Akhmed Zakayev

The case for: Prominent critics of Ramzan Kadyrov, Chechnya's Prime Minister and aspiring President, have been killed in recent weeks, including Politkovskaya and Movladi Baisarov, head of a Chechen commando group. Baisarov, who had fallen out with Kadyrov, was gunned down in Moscow on Saturday. Kadyrov is backed by Putin and Litvinenko's alliance with Zakayev made him vulnerable.

The case against: Litvinenko posed no direct threat to Kadyrov's regime and his key criticisms were directed against the war launched by Putin. He had accused Putin of responsibility for killing Politkovskaya, but so had others. While Chechnya remains a potential Achilles' heel for the Kremlin, Litvinenko's influence from London was minimal.

Rogue Russian operation

The theory: Litvinenko’s former colleagues in the Federal Security Services (FSB) wanted to avenge his betrayal of the service. Acting on their own — but in the belief that they were helping the Russian President — they arranged the assassination using a special poison developed in a top secret laboratory.

The case for: Litvinenko was hated by the FSB for exposing an alleged plot to kill Berezovsky and for accusing their agents of blowing up apartment blocks in Moscow in 1999 and then blaming it on Chechen terrorists. They have expertise in special operations and were authorised by parliament this year to eliminate enemies of the state abroad.

The case against: The Russian security services are tightly under Putin’s control. Assassinating a British citizen in London would be a momentous decision. Officers serving in the Russian armed forces have a strong tradition of not taking initiatives without approval from their superiors.

Natural causes or suicide

The theory: Obviously now a redundant theory, but while doctors were unable to explain the cause of Litvinenko’s illness — with thallium or a poison cocktail lines of inquiry — there were suggestions that he might have poisoned himself, been exposed to radiation or killed by a dose of chemotherapy.

The case for: It took doctors weeks to grasp the severity of his condition. Even though traces of a radioactive substance were found in his urine, at his home and in several places he had visited, there is still no indication of how it entered his body.

The case against: Quite apart from the discovery of polonium-210, it is almost unheard of that a perfectly fit man falls ill and dies three weeks later of a natural disease or a toxic compound that doctors and scientists cannot identify.

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