Litvinenko died in 2006 after being poisoned with Polonium-210, a highly radioactive substance.
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Russia was responsible for the murder of ex-spy Alexander Litvinenko, the European Court of Human Rights (ECHR) ruled on Tuesday.
The court said there was a "strong presumption" that Litvinenko had been assassinated by individuals "acting as agents of the Russian state".
It also added the Russian government "failed to provide any other satisfactory and convincing explanation of the events", nor did it carry out "an effective domestic investigation" aimed at identifying and punishing those responsible.
Litvinenko had worked for Soviet and Russian security services before defecting to the UK, where he was granted asylum in 2001.
In November 2006, he died in London after being poisoned with Polonium-210, a highly radioactive substance.
A previous inquiry carried out by British authorities in 2016 found the murder had been carried out by Russian nationals Andrey Lugovoy and Dmitriy Kovtun. Moscow has always refused to extradite them.
On Tuesday, Russia said it would not recognise the "unfounded" ruling of the Strasbourg court.
"So far, the investigation has yielded no results, so making such allegations is at least unfounded," Kremlin spokesman Dmitry Peskov told reporters.
"It is unlikely that the ECHR has the powers or the technical capacity" to shed light on the case, Peskov added.