Litvinenko's widow points the finger at Russia

The radioactive poison used to murder the former KGB operative Alexander Litvinenko "almost certainly" came from a state-controlled plant near Moscow and could not have been obtained without the knowledge or complicity of the Russian state, it was claimed yesterday.

Lawyers representing Mr Litvinenko's widow, Marina, said a leading international nuclear expert had provided testimony that it was "very likely" the poison, a highly toxic isotope called polonium-210, was made at the Avangard nuclear facility. The testimony is part of a lawsuit filed this week against Russia in the European Court of Human Rights (ECHR).

The announcement was made as family and friends of Mr Litvinenko marked the first anniversary of his death on 23 November last year, three weeks after he swallowed a lethal dose of polonium-210 in a cup of tea in the Millennium Hotel in central London where he was meeting three Russian businessmen.

One of those businessmen, Andrei Lugovoy, who is also a former KGB officer, has been identified by Scotland Yard as the chief suspect for the murder. The refusal of the Russian authorities to allow Mr Lugovoy to stand trial in London has contributed to a sharp deterioration in relations between London and Moscow.

Marina Litvinenko, who was herself affected by the poison after her husband returned home from the hotel meeting on 1 November and fell ill, said she believed it was now unlikely that diplomatic efforts to see Mr Lugovoy extradited will succeed and she wanted to bring her fight for justice to a wider international arena.

Mrs Litvinenko joined Boris Berezovsky yesterday at a packed press conference close to University College Hospital, where Mr Litvinenko died. She said: "I lost my husband and I want to know who was behind the killing. I promise we will definitely find out who is responsible for this. Without this knowledge we cannot feel we are safe."

The case presented to the ECHR claims that Mr Litvinenko – a former officer in the KGB and its successor, the FSB, who fled Russia for Britain in 2000 after alleging Moscow's involvement in atrocities against its own people – was killed with the complicity of the Russian government and that authorities have failed properly to investigate the killing.

Louise Christian, a solicitor who has represented Britons held in Guantanamo Bay, said the file presented to the European court included a report from a UK-based professor of theoretical physics who is familiar with the Avangard plant, which for decades was the hub of the Soviet Union's nuclear weapons production inside the closed city of Sarov.

The scientist concluded the plant was "highly likely" to be the source of the particular form of polonium-210 used to kill Mr Litvinenko because the isotope was produced in very few locations around the world and the purity of the sample used in London pointed to Avangard as the source.

Ms Christian said: "In the words of our expert, if that is the case then it is almost certain that the Russian state or its agents were responsible for the poisoning. ... Security at Avangard is still very good and very tight and polonium-210 of this naturecannot be removed without the knowledge or connivance of the Russian authorities."

Polonium-210 emits alpha radiation and is lethal if swallowed or breathed in but relatively benign outside the body. It is manufactured for anti-static commercial applications at Avangard for export to the US but could not be converted into poison from this, Ms Christian said.

The lawyer added that Mr Lugovoy, who made a fortune in the soft drinks business and is standing for a seat in the Russian parliament next month, was singled out by the Metropolitan Police investigation because he was alone with Mr Litvinenko in a bar at the hotel when the polonium-210 was administered.