It is the perfect place to meet a man from the KGB. Boris Karpichkov – former KGB operative and double agent – suggests we meet under the shadow of Marble Arch in central London. I am late. But he is easy to spot: a gaunt, thin, pale figure with the slightly haunted look of someone who has spent their career in the twilight world of espionage.

Since fleeing to Britain in the late 1990s Karpichkov has preferred to keep a low profile – unlike another, better known Moscow agent who fled to London, one Alexander Litvinenko. Now, with the KGB's most famous graduate, Vladimir Putin, about to get his old Kremlin job back, can Karpichkov shed light on the murky world of Russian spying?

Born in 1959 in Soviet Latvia, Karpichkov grew up in a patriotic communist family and became a mechanical engineer. The KGB approached him when he was working in a factory making parts for the aerospace industry. He enrolled at the KGB's academy in Minsk in 1984, learning, among other things, how to shoot, and how to kill with his bare hands. He was assigned to the Riga branch of the KGB's prestigious Second Directorate, specialising in counter-intelligence. He reached the rank of Major.

After the Soviet collapse, Karpichkov stayed in Latvia, now independent and at odds with Moscow, and joined Latvia's new intelligence service. Secretly, however, he continued to supply information to the KGB – renamed the Federal Security Service or FSB.

For three years he was a classic double agent. He says he broke into and planted bugs in the British embassy in Riga. He ran audacious disinformation operations against the CIA. He still has the tools of his trade: skeleton keys used for breaking into the flats of targets (small pieces of metal that might be mistaken for a bicycle repair kit), and a wide-range "scanner", which looks like a chunky walkie-talkie, for eavesdropping.

But in 1995 Karpichkov's problems began. He grew unhappy with the increasingly corrupt FSB, which, he says, failed to pay him. The Latvians began to suspect, correctly, that he was working for the Russians. Back in Russia with his cover blown, he spent several months in a Moscow prison before slipping into Britain on one of the false passports he was given as a KGB officer. He hasn't been back to Russia or Latvia since.

In exile in Britain, Karpichkov has written a colourful memoir about his time in the KGB, for which he is now seeking a publisher. In it, he recounts his own clandestine adventures – operations involving psychotropic drugs, an order to kill (he says he didn't carry it out) and the time when one of his targets – a visiting Japanese military attache – realised he was under surveillance. The attache erected a small tent in the middle of his hotel bedroom. "He was well trained," Karpichkov says. Standard FSB protocol was to hide bugs everywhere, he says, including in the bathroom and the bedroom: "Our motto was to know everything." East Germany's notorious secret police, the surveillance-obsessed Stasi, used the same slogan.

Britain is an obvious target for Russian espionage, given the large number of Russians who live and work here. After defecting, Karpichkov needed a new outlet for his talents. He mounted undercover commercial investigations into several high-profile Russians living in London including, he says, Boris Berezovsky and Roman Abramovich. He also found himself drawn to the Kremlin's overseas spying operation in London. At one point he infiltrated a pro-Putin youth group, Marching Together (later renamed Nashi), as it sought to undermine and discredit Russian exiles opposed to the Putin regime. The group's key targets included Litvinenko, the Chechen separatist leader Akhmed Zakayev and Boris Berezovsky, the oligarch and Putin critic embroiled in a colourful high court litigation battle with Roman Abramovich.

Karpichkov agrees with the head of MI5, Jonathan Evans, that the number of undeclared Russian intelligence officers in London hasn't decreased since the end of the cold war. "Britain is a prime target for Russian intelligence services," Karpichkov says. In the years after 9/11, Britain's security services turned their attention to combating Islamist extremism. They assumed that the Russians would do the same, and scale back their operations in the UK.

They didn't.

But it is an open question how successful much Russian espionage actually is. The Russian deep-cover agents exposed in the US in 2010, including the glamorous Anna Chapman, now a cheerleader for Putin's United Russia party, seemed more bungling than sinister. Similarly, many of the Kremlin's British operations, apparently aimed at influencing public opinion, come across as primitive and ridiculous.

In 2004, Karpichkov met two Nashi representatives sent by Moscow to London – "Alvis" and "Irina". He didn't reveal his real identity. "Alvis" and "Irina" paid demonstrators £4.50 an hour in cash to take part in various pro-Kremlin rallies in London: one against Zakayev, another against Anna Politkovskaya, murdered two years later by an unknown hitman in the stairwell of her Moscow apartment.

According to Karpichkov, Nashi also targeted the actor and political activist Vanessa Redgrave, a close friend and supporter of Zakayev's. In late 2004 the pro-Kremlin group organised a fake "demonstration" in a park close to her Chiswick home. That evening unknown intruders damaged her front door. Redgrave complained to police. Karpichkov alleges that the damage was caused by elements within Nashi. He also claims Kremlin activists were behind an October 2004 attack on the neighbouring Muswell Hill homes of Litvinenko and Zakayev; someone lobbed molotov cocktails into their gardens.

And what of Litvinenko's polonium murder in November 2006, killed by a cup of radioactive tea? Karpichkov says he has no doubt that his old employer, the FSB, was behind it. He also says that he warned the British intelligence services via intermediaries that Litvinenko's life was in danger. The agency politely ignored his advice, he says – a "negligent" decision that allowed a Russian hit squad to kill him.

Karpichkov also claims that a Russian diplomat based at the Russian embassy in Kensington, west London, was involved in the Litvinenko operation and died mysteriously afterwards.

The three men who allegedly poisoned Litvinenko – Andrei Lugovoi, Dmitry Kovtun and Vyacheslav Sokolenko – are all former members of the KGB's Ninth Directorate. According to Karpichkov, other KGB departments regarded "niners" as little more than jumped-up bodyguards, providing protection to top Kremlin officials. He believes someone else – an unidentified fourth person – flew to London to mastermind Litvinenko's poisoning. They then flew out again. "It [an overseas assassination] is quite a sophisticated process. They would have needed someone with brains," he explains.

These days Karpichkov tries to keep a low profile. He writes, stays in touch with events in Russia and vanishes now and again on mysterious trips whose purpose he declines to explain. But, I suspect, he is never able to relax.

Unsurprisingly, Russian agents working at the London embassy keep an eye on him, he says. There have been anonymous death threats, bugging and cars with Russian diplomatic plates repeatedly parked outside his London flat. Old habits of suspicion are hard to shake.

After we meet at Marble Arch, we go to a nearby cafe; Karpichkov, however, suspects that someone sitting at the next table is listening in; we leave and stroll instead among Hyde Park's plane trees. Karpichkov is used to exile now but my sense is that even after a decade in London he has to live by Moscow rules – to accept that he will never be able to relax fully, or to forget his old instincts and training in counter-surveillance. We exchange emails over a period of several months after our initial meeting. Karpichkov, it turns out, knows a huge amount: about Russia's murky arms sales abroad, for example. He is intelligent, and a first-class analyst – but, of course, he has no one to report to.

Karpichkov says he is "no way scared". But he confesses he is now "dead tired" of the exhausting world of espionage, and concerned for the safety of his wife and grownup children. It strikes me that his story has little in common with the glamorous fantasies of James Bond. It is more similar to the drab, amoral universe of John Le Carre, with its lack of heroes, and atmosphere of slow, psychological attrition.

And what of rich Russians buying up British institutions? Chelsea FC, Waterstones, the Evening Standard and Independent newspapers are all now owned by wealthy Russians. Karpichkov is scathing about Alexander Lebedev, the billionaire turned British press baron, who worked as a KGB spy in London during the Gorbachev period. Lebedev's son, Evgeny, acquired British citizenship in 2010 and lives in London. "Lebedev pretends he is a liberal in Russia," Karpichkov says. "But we have a saying that he [Lebedev] will never piss against the wind." The wind of course is Putin. Putin, meanwhile, is certain to win Russia's presidential "election" next month, despite the biggest protests against his rule since the fall of the Soviet Union. So far he has made few concessions to the demonstrators, whom he has dismissed as western stooges.

As for Karpichkov, his application for asylum was rejected by the authorities, and it was only in 2010 that he finally got a British passport and permission to remain, a decade after he arrived in the UK.

Karpichkov says he is unimpressed by the way that MI5 last year tried to deport Katia Zatuliveter, the 26-year-old parliamentary aide and lover of the Liberal Democrat MP Mike Hancock. The home secretary Theresa May accused Zatuliveter of spying for Moscow, and tried to deport her on the grounds she was a threat to national security. "They looked like stupid little amateurs," he says of MI5, which failed to convince a special tribunal that Zatuliveter was shipping British secrets to Moscow. "If you are trying to prove a case you need to have strong evidence," he says, adding that he doesn't think Zatuliveter was ever a "classic staff agent". Nor does he believe she was on the payroll of the FSB, or the SVR, Moscow's foreign intelligence service. (The two agencies are fierce rivals, with the FSB authorised by Putin to conduct its own secret missions – and assassinations – abroad.)

We part in Hyde Park. No one appears to have been tailing us. There are no figures skulking behind the trees. But I find myself glancing over my shoulder, just in case.