One year on: Widow of murdered spy vows to bring killer to justice as radioactive poison is traced back to Russia

Mr Litvinenko, a former KGB agent, died a year ago today after he was poisoned in London with polonium-210 placed in his tea.

Speaking at a press conference, Mr Litvinenko's wife, Marina, said: "I promise we will find who is responsible for this. Without this knowledge, we just cannot feel we are safe."

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She confirmed she and supporters were taking their case to the European Court of Human Rights spurred on by the fact an American nuclear expert believes the radioactive substance used to kill Mr Litvinenko is "almost certain" to have come from Russia.

Legal papers were filed yesterday accusing the Russian government of complicity in the murder and of failing to carry out a proper investigation into the death.

Mrs Litvinenko accused Mr Putin of protecting and endorsing her husband's killer.

Scotland Yard has identified Andrei Lugovoi as the main suspect in the polonium plot but Russia refuses to allow Britain to extradite him to face charges.

Mrs Litvinenko said: "By doing so, Mr Putin has tainted his office, his government and himself with this horrendous crime. He turned a murderer into a national symbol.

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"At the very least, this makes him an accessory after the fact. And it adds credence to my husband's last statement alleging that it was Mr Putin who ordered his murder in the first place."

Mrs Litvinenko said she still hoped the British authorities would succeed in extraditing Mr Lugovoi to face justice in London.

She added: "It's still a very personal case for me. I lost my husband and I want to know who was behind the killing."

Solicitor Louise Christian said an American nuclear expert had traced the radioactive isotope used to poison Mr Litvinenko to the Avangard plant in Russia.

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She said the expert believed that it was "almost certain" the Russian state was behind the poisoning because the substance was kept in such high-security conditions.

She said: "We believe that, if the Russian government were serious about this matter, they would be co-operating with the British investigation and the request for extradition."

Ms Christian warned that the European court action would be a "long and drawn-out procedure" which could take many years.

She added: "At the point where we decide there is absolutely no hope of Mr Lugovoy being brought to justice, then Marina will be entitled to ask for an inquest."

Mr Litvinenko's father, Walter, paid an emotional tribute to his son today as he joined by his son's widow, Marina, and other family and friends outside University College Hospital in London where Mr Litvinenko died.

They marked the murder of the outspoken Russian dissident by re-reading his deathbed statement, accusing President Vladimir Putin of complicity in the plot.

Speaking with barely contained anger in his voice, Mr Litvinenko accused Andrei Lugovoi of being the "chief executioner" in the polonium plot.

But he said: "The gangsters who poisoned my son in the Millennium restaurant here are even today trying to show the whole world that they can get away with anything.

"As a reward, the chief executioner of my son, Mr Lugovoi, has now been given a seat in the Duma.

"The main executioner, Mr Putin, is afraid to leave his position, as he wants to maintain his power in order to cover up this crime."

He added: "Western leaders and European leaders must take these things extremely seriously, because the gas and oil they buy from Russia may turn into something rather more sinister.

"I trust my son did not die in vain, and the truth and justice for which he was fighting will prevail in the whole of the world, including Russia."

Mr Litvinenko's friend, Alex Goldfarb, read out his deathbed statement, watched by billionaire supporter Boris Berezovsky.

He repeated the memorable words in which the exiled Russian told Mr Putin the "howl of protest" will echo in his ears forever.

He said: "May God forgive you for what you have done, not only to me, but to beloved Russia and all of its people."