Deep in the wooded heart of London's Highgate Cemetery, Marina Litvinenko slips on a pair of gloves, fills a metal vase with fresh water and kneels to arrange red carnations on her husband's grave. The flowers make a vivid splash of colour against the surrounding dark green of ivy and holly. Dressed in a soft leather jacket and tweed skirt, she sits back on her heels to survey the result. A framed photograph of Alexander Litvinenko leans against the headstone; a little weather-worn now after several years in its damp woodland setting, it is nonetheless clear that this fair, pleasant-looking man in the prime of life bears little resemblance to the image that flashed around the globe in November 2006: the hairless skull propped on pillows, a green hospital gown open across a bare chest studded with electrode patches, and the heavy lidded, helpless gaze of a man who knows he is dying.

In October 2000, Lieutenant-Colonel "Sasha" Litvinenko of the Russian Federal Security Service (FSB) - successor to the KGB - fled his homeland in fear for his life and sought asylum in Britain. Almost exactly six years later he lay gravely ill in an isolation bed in University College Hospital, poisoned by a deadly radioactive isotope, polonium-210 - a substance so rare and difficult to detect that it was only identified just hours before his death by analysts at the Atomic Weapons Establishment in Aldermaston.

A grain of sugar contaminated by polonium-210 is enough to kill a man, but is only active once ingested. Police investigators wanted to interview two ex-KGB officers who were believed to have administered the poison while drinking tea with Litvinenko at London's Millennium Hotel. Traces were found on crockery and in the hotel rooms of the two men who returned to Russia shortly after the tea date. Application by the Crown Prosecution Service for their extradition was refused by Russian authorities.

Marina Litvinenko has waited eight years for the truth about her husband's death. Various alternative theories have been proposed, including that he had imported polonium-210 himself for some unspecified purpose and accidentally ingested the poison. In 2011, an inquest was opened but did not go ahead because, according to the coroner, it would be impossible to get to the bottom of what had happened without access to secret papers being withheld by the British government. A second coroner insisted that only a statutory inquiry would have the power to examine serious allegations of Russian state involvement in Litvinenko's death. Still, Home Secretary Theresa May and Foreign Secretary William Hague held out, saying the release of such sensitive information would jeopardise national security, and an inquest should go ahead without it. But their real concern was more likely to be about damaging relations between Britain and Russia: the coroner was being "steamrollered by two states acting in collaboration with one another", says Ben Emmerson QC, acting for Marina.

Then in February this year came a breakthrough: three senior High Court judges stopped short of ordering an inquiry, but ruled the Home Secretary would have to come up with better reasons for not holding one. After the hearing, a relieved Marina Litvinenko appealed to Theresa May, "as one woman to another", urging her to consider "how she would feel in my position". The killing of her husband "was the murder of a British citizen on the streets of London using radioactive poison", she said. "You would have thought that the government would want to get to the bottom of who was behind it."

We meet at noon on an early spring day at the locked gates of Highgate's west cemetery. Marina presses a bell and a porter lets us in, greeting her warmly. Tourists are only allowed in to this side of the cemetery on a guided tour. It is peaceful here. A good friend of the Litvinenkos secured the burial plot in this coveted spot. "Every time I come in I say thank you to him," says Marina, "because you cannot imagine a better place to be, to visit and know Sasha is safe."

The funeral service was a humanist one because, Marina explains, although her husband had been a Russian Orthodox Christian all his life, he converted to Islam on his deathbed. "He asked [his close Muslim friend] Akhmed Zakayev, 'How do you talk to your God?'" recalls Marina. "Then he said he wanted to be Akhmed's brother and convert." She says when her father-in-law came to the hospital he brought some Russian icons. "Sasha told him, 'No Daddy, I'm a Muslim now.' His father said, 'That's okay, at least you're not a Communist.'" She laughs. "Later an Imam came to the hospital and said some prayers, though Sasha was by then already unconscious. Akhmed told me, 'I had to do it Marina, even if you don't like it.' But I said it is Sasha's choice." Did she mind? "No, I am Buddhist, very accepting."

Marina stops at the intersection of two paths. "Here is Sasha." Because of the deadly nature of the poison that killed him, her husband's body is encased in a secure capsule inside his coffin with an order that it should not be touched or opened for 28 years. The grave is surrounded by a carpet of wood anemone and fading snowdrops; the pink granite headstone bears the name "Sasha" and the line, "To the world you are one person but to one person you are the world."

Marina and Alexander grew up in the Soviet era: she, the only daughter of hardworking parents, an earnest Pioneer (Communist girl scout) who became a competitive ballroom dancer; he, the only son of divorced parents, raised by his grandparents, who enlisted in the army at 17. When the couple met, both aged 31, he was an officer in the KGB and in an unhappy marriage with two children; she was an aerobics instructor.

A few weeks after they met, they went for a walk. "I was so tired," Marina tells me. "We sat on a bench and Sasha told me, 'Take off your shoes', and he massaged my feet. I was shocked, it was so intimate." Within four months she was pregnant. "He was so happy; he said, 'Now you won't leave me'." Alexander divorced his wife in 1994, and married Marina later that year.

During their years in Russia, Marina rarely knew where her husband was or when he would be home; his work was secret and he spoke little about it. If she was worried, she learnt not to show it. All she knew was that he was loyal and honest."He helped people who had been kidnapped or intimidated," she says."He was one of a new, bright, uncorrupted generation of officers. He loved his work with the FSB - it was like his family."

But in the Byzantine world of Russian political plotting and the post-Soviet scramble to capitalism, the integrity Marina ascribes to her husband was not only not prized but would lead to his downfall. In 1997, he had been assigned to URPO, the highly secretive "Division of Operations against Criminal Organisations" and, shortly after Vladimir Putin was appointed director of the FSB, Litvinenko went to alert him to corruption inside the agency. When his warnings were ignored, he gave a press conference together with fellow whistleblowers detailing their claims. They included an order he had received to kill the oligarch Boris Berezovsky, a hugely influential man and kingmaker to Putin, now evidently out of favour.

For an operative from the covert URPO to stick his head above the parapet like this was bold indeed, and Putin was furious. "Officers should not stage press conferences," he told the Kommersant newspaper.

In March 1999, Litvinenko was arrested on charges of exceeding his official powers and "causing bodily harm to a suspect" and imprisoned. Nine months later he was cleared of both charges, but immediately re-arrested on further charges and released on bail. The family remained under surveillance until July 2000, when a new case was opened and further charges brought. By now Litvinenko knew the waters were closing over his head and planned his escape from Russia, advised by his old ally Boris Berezovsky, now himself in exile in Paris after surviving several attempted assassinations.

Marina knew nothing of her husband's plans until she found herself on the run through Turkey with him and their six-year-old son, Anatoly. They were accompanied by Alex Goldfarb, a Russian human-rights activist based in the US who had been summoned to aid the fugitives. "Alex was keeping notes," says Marina. "Sasha said, 'Oh Marina, this is going to be like a Hollywood movie.'" She was not so thrilled: "I didn't want to leave Russia and my mother," she says, "but Sasha told me, 'If we go back I will be imprisoned or killed and you and Anatoly will have a difficult time.'"

But in London, while Marina busied herself creating a new home and making friends at the school gates with Anatoly, it was Sasha who missed his old life. He watched only Russian movies, immersed himself in defence of Chechnya against Russia's "dirty war" and wrote two books accusing the Russian secret services of terrorism. Did Marina resent this obsessiveness? She gives a tiny shrug: "My duty was to make a balance in the family. I couldn't change him. We supported each other like this" - she makes a steeple of her hands. "At the time I thought some of what he wrote about corruption in Russia was too radical, but now many things have turned out as he predicted."

Death of a Dissident, the book Alex Goldfarb co-authored with Marina in 2007, argues that Litvinenko's death was authorised at the highest level. Does Marina really believe President Putin ordered her husband's murder? She gives a wry smile: "In Russia, nobody needs to make an order; they just say something like 'I hate this person' and it is understood."

Does she hate Putin? "I don't have hate for him, I have disgust." The day Alexander died, a member of the Duma (Russia's parliament) commented: "The deserved punishment reached the traitor ... In Russia they do not pardon treachery." That night, police came to the Litvinenko home and told Marina she should leave the house, which might be dangerously radioactive. Was she frightened? "The police offered us a safe house outside London but I said no, I want to have a normal life." Twelve-year-old Anatoly was then a pupil at City of London School, a private selective school for boys, the fees paid by their old friend Berezovsky. "At first I thought maybe Anatoly should stay at home, not go to school," she recalls. "But then we received a letter from his schoolmates saying, 'We miss you, we want you to come back.'" She smiles as her eyes fill with tears: "I still have that letter."

Now 19, Anatoly is studying for a degree in politics. He has dropped the name of Anthony Carter, which he was given when the family sought asylum in Britain, and reverted to Litvinenko. "He is proud of his name and of his father," says Marina. But after his death, Alexander Litvinenko's name became the subject of a media smear campaign in his native Russia.

His ex-wife and children flew to England for his funeral, and at first Marina believed she and they were united in mourning the man they had loved.

"I thought we could make a step towards each other," she says, "but later they started to blame Sasha for being a bad Russian citizen, and gave interviews on TV criticising him. And it was the same with Sasha's father; he had been so supportive and was with him when he died, then two years ago he started to call him a traitor. He even asked Putin to forgive him." She looks at me. "It was devastating. Anatoly said, 'I don't have a grandfather any more.'"

We walk in silence for a moment, back towards the gate, past vaults and catacombs and Gothic monuments, past the grave of a descendant of the Russian composer Prokofiev, flanked by urns of white lilies. "This name of Sasha," she continues, "I don't like to be used in a bad way - to call him a traitor. People made up so many things. I thought, no, I must stop this."

Marina has not struggled alone: friends have been steadfast in their support, her legal team works for nothing. "I told Ben [her barrister] that we had no more money to pay his fees," she says. "He told me, 'Marina, I promise not to leave you. I will be with you to the end. If we get to the point where there is no more we can do, I will tell you.'"

It is not difficult to see why this dignified, plucky woman attracts such loyalty. Each person we encounter greets her with affection; at a cafe near the cemetery the rather mournful-looking proprietor asks how she is, enquires fondly after her son, and refuses payment for our coffees. Settled at a table outside in the spring sunshine, she says she is hopeful February's court decision will persuade the government to grant an inquiry. "I am sad that the British state has been against me in this case," she says. "I don't want to make an enemy of Britain or of Russia, but we need to understand what happened to Sasha and why. We deserve to have some sunlight on this dark situation."

Marina has no plans to return to Russia. She has been preoccupied with her legal battles, she says, but in the future would like to work for a cancer charity, "because good friends have died from cancer and because people seem to draw some strength from me".

Meanwhile, she teaches dance and takes dancing classes herself to keep fit. Has she met anyone? She smiles: "Never say never. I know Sasha would be happy if he could see me happy. I feel he is my guardian angel - if I was with someone else they would have to appreciate that Sasha is here with me, too."