The widow of former spy Alexander Litvinenko last year is seeking a ruling from the European Court of Human Rights that the Russian state was complicit in his murder.
Marina Litvinenko's lawyer, Louise Christian, said she had obtained expert evidence that it was "highly likely" that the radioactive polonium that poisoned Mr Litvenoko in London had come from Russia's Avangard plant - a state facility surrounded by tight security.
"We say that there is evidence . . . of either active complicity or connivance by the Russian Federation in the murder of Alexander Litvinenko," Ms Christian told a news conference on the first anniversary of Mr Litvinenko's death.
The former KGB security officer, who became a fierce Kremlin critic in exile, was poisoned by polonium slipped to him in a cup of tea. He died a painful death after three weeks in a London hospital.
From his deathbed, Mr Litvinenko accused Russia's President Vladimir Putin of having a role in his murder.
Britain wants to prosecute another ex-KGB man, Andrei Lugovoy, for the murder. But Russia strongly denies state involvement and refuses to hand him over on constitutional grounds. The case has badly damaged diplomatic relations between the two countries.
Earlier today, Mrs Litvinenko joined her late husband's friends and father for a short commemoration in front of the London hospital where he died.
Walter Litvinenko branded Mr Putin an "executioner" and said: "I hope my son didn't die in vain and this villain will answer for everything."