If you wanted to make the death look natural, or just to keep things simple, you would presumably avoid the restaurant scenario. And yet, if many Russia-watchers are to be believed, the country's Federal Security Service (FSB) recently attempted just such an assassination.

On November 1, Alexander Litvinenko, a 43-year-old Russian who used to work for the FSB, (the post-Soviet version of the KGB), had lunch at Itsu, a cheap-and-cheerful Japanese eatery, with an Italian spycatcher. By that evening, he was feeling so ill he was admitted to hospital. Doctors wasted 10 days trying to treat him for food poisoning. His condition deteriorated - hair falling out, difficulty speaking, white blood cells disappearing, unable to eat, even nourishment from a drip causing him to vomit. It was only when they listened to his pleas to investigate whether he had been poisoned that doctors realised Litvinenko's body contained three times the fatal dose of thallium, a tasteless, odourless killer used in rat poison until, in the 1970s, it was banned as too dangerous. They are now trying to neutralise the slow-acting poison; but it may be four weeks before it is clear whether the ex-secret service man will live.

Litvinenko's friends in London have been quick to accuse the Kremlin of being behind this poisoning. They say Russia wanted to stop Litvinenko investigating the assassination last month of another high-profile critic of the Russian government - his friend, the campaigning journalist Anna Politkovskaya. They believe the Kremlin was also to blame for Politkovskaya being shot outside her Moscow apartment door.

The first British instinct is to laugh off these accusations. For Litvinenko's friends in London are a coalition of avowed enemies of the Kremlin. They range from a London-based human-rights activist called Alex Goldfarb to a London-based envoy for separatists fighting Russian rule in Chechnya, Akhmed Zakayev (Litvinenko's neighbour in north London), to the exiled and London-based oligarch, Boris Berezovsky, the highest-profile opponent of President Vladimir Putin, in whose Mayfair offices the rest of the group can often be seen. Berezovsky and Zakayev won political asylum in Britain after the Russian government failed to prove its claims that there was a case for both men to face criminal charges (in Berezovsky's case, for fraud; in Zakayev's, for war crimes) at home.

But there is a precedent for Litvinenko's poisoning. In 1978, operatives from one of the Soviet Union's satellite states, Bulgaria, decided to bump off Georgy Markov, a diplomat who had defected to Britain. In true James Bond fashion, his assassin prodded a ricin pellet under the defector's skin from the point of a doctored umbrella while he stood in a bus queue. Markov, who felt a sharp pain as the pellet entered his body, died after three days. When Bulgaria's Communist regime collapsed a decade later, a stock of special assassination umbrellas was discovered at the interior ministry in Sofia.

A cultural preference for the more absurd kind of murder has run through stories of KGB activity over the years, in keeping with the agency's fondness for smear and destabilisation campaigns in the countries in which it operated while the Soviet Union was a worldwide superpower. Even today, foreigners who deal regularly with Russia quickly learn to give at least some credence to the regular dark stories of dirty tricks by murky and usually unnamed "forces".

Many of the latest FSB stories have a London component because, since the 1990s, Britain's oligarch-friendly tax laws have made London the home-in-exile of choice for any Russian with money to burn. It is not just Berezovsky, with his homes in central London and a manicured high-security estate in Surrey. London has become home to so many Russians that it is known jocularly as Moskva na Temze - Moscow-on-Thames - and the streets of Knightsbridge, Mayfair and Belgravia ring with the voices of wealthy Russian ladies who lunch. British public schools and universities are full of posh Sashas and Pashas. With more Russians than ever before packing into our country, espionage levels are said to be at their highest, too, to monitor the newcomers.

In 2006, any story accusing the Russian secret services of dark deeds leads in one direction - to President Putin, an ex-KGB man whose job before he started running Russia in 2000 was running the FSB. All such allegations are intended to prove that Putin has given the FSB back almost all the powers of the Soviet KGB.

Litvinenko's and Berezovksy's friends, together with the actress and activist Vanessa Redgrave, have spent the past few years campaigning to prove that Putin came to power as a result of a deeply cynical FSB plot in the autumn of 1999. This, they say, involved blowing up apartment blocks all around Russia, wrongly blaming the attacks on Chechen separatists, and playing on Russian fears of the fierce Muslim Chechens both to start a new war in Chechnya and to win Putin the presidential elections. Litvinenko's book, Blowing Up Russia, details this claim. The ex-operative, who Russia says was removed from the FSB for corruption, claims he left because he did not want to carry out an FSB order to assassinate the man who was then Putin's political Enemy Number One - Berezovsky.

Moscow, in turn, hints that some of the more exotic crimes sometimes laid at its door are the work of "foreign connections". These hints seem to suggest that Berezovsky might be mischievously talking up a series of unconnected events into a froth of imaginary conspiracy.

Until now, this web of allegation and counter-allegation has been thought simply too strange to win many headlines in the British media. Yet now the British public is becoming uneasily aware of just how many other lurid stories there are about poisonings, shootings and helicopter accidents afflicting enemies of the Kremlin worldwide.

In the winter of 2004, the pro-western candidate for Ukraine's presidency, Victor Yushchenko, was poisoned with dioxin, a drug that thickened his film-star features into an elephant-man mask and nearly killed him. The poisoning has often been blamed on pro-Moscow secret service operatives. In the autumn of 2004, Politkovskaya, the journalist, said she had been poisoned aboard the plane she was taking south to the Chechen frontier, hoping to help negotiate a peaceful end to the hostage drama at Beslan school. In the summer of 2004, a Chechen separatist leader called Zelimkhan Yandarbiyev was assassinated in Qatar; two Russians were arrested for the killing, though Moscow denied any connection. Earlier that summer, as Putin jailed Russia's richest oligarch, his political opponent Mikhail Khodorkovsky, on fraud charges that the billionaire says were politically motivated, a helicopter carrying Khodorkovsky's British lawyer, Stephen Curtis, crashed on the English south coast. Curtis had said shortly before the crash that if he died mysteriously, "it would not be an accident".

I was at the London party where Politkovskaya shocked the British human-rights great and good by describing how she was slipped a knock-out drug on the plane south. She woke up in hospital several days later - too late for the Beslan children, who had by then been killed in their hundreds in a botched rescue attempt. What she remembered of the experience was the three men she had noticed in the plane, staring at her with the "eyes of enemies". She blamed the Russian secret services for poisoning her. It was impossible not to believe what this fearless truth-teller said.

It was in the 1990s that Russia's super-rich began to take an interest in Britain, as they fell out with the powers that be in Moscow and found London a friendly home. Since Putin came to power in 2000 and began crossing swords with more and more of the irrepressible oligarchs who had got too rich and powerful under his predecessor, Boris Yeltsin, all wealthy people who fear they too might fall foul of the Kremlin - even those who, like Roman Abramovich of Chelsea football club fame, are currently on friendly terms with Moscow - make a point of having a bolt-hole over here.

What is most instantly noticeable about this immigration is the way it is pushing up London house prices way beyond the apparent logic of what the market can sustain. To the great joy of estate agents, rich Russians like to knock several lofts together to make enough lateral space for themselves and their staff and the expensive simplicity of their high-end interior decor; they are happy to drop millions for the right living space.

Russians are taking over the gossip columns, too, as they become a flamboyant and accepted part of London life. Photographers love Natalia Vodianova, Russian model wife of socialite Justin Portman. The married Abramovich's "friendship" with the pretty student daughter of another Russian billionaire with a London home has hogged plenty of inches in recent weeks, as have his occasional London football parties at Chelsea for his Moscow entourage and their visiting wives, all in fur coats and glittering with diamonds. Even the older, less photogenic Berezovsky, who makes his money these days by trading in London property, has a glamorous young wife and the pair are snapped at society events such as days out at Goodwood, in hats and tail-coats, escorted by the PR guru Tim Bell.

But the reporting of the Litvinenko case has brought the darker side of the Russian picture into focus. It brings the British, for the first time, within elbow-rubbing distance of the political struggles of Russians. This week's glimpse at the ugly underside of Moscow politics has been enough to jolt a lot of people here out of their innocent British belief that conspiracy theories are only for crackpots. Is it now time to start being frightened that the deadly and secretive skirmishes that have come to characterise life in Russia are spilling over into the UK?