"The UK Government judges that the Russian Government, specifically the Russian military, was responsible for the destructive NotPetya cyber-attack of June 2017," Foreign Office minister Tariq Ahmad said in a rare direct rebuke.
The ransomware attack contaminated thousands of computers worldwide, particularly affecting multinational companies and critical infrastructure, such as radiation monitors at the old Chernobyl nuclear power plant and the ports of Mumbai and Amsterdam.
Companies hit included the Russian oil group Rosneft, Danish shipping company Maersk, US pharmaceutical giant Merck, French construction specialist Saint-Gobain and the British advertising firm WPP.
Ukraine was the worst-affected country, with banking operations compromised in what authorities said was an unprecedented attack.
"The attack masqueraded as a criminal enterprise but its purpose was principally to disrupt," said the British government.
London has taken an aggressive stance against Moscow, with Prime Minister Theresa May last year accusing it of "seeking to weaponise information" and condemning "the illegal annexation of Crimea".
British army chief Nick Carter later said that Russian cyber-warfare presented a direct threat to Britain and called for more investment in the armed forces to be able to deal with it.
Minister Ahmad said the Kremlin had "positioned Russia in direct opposition to the West", but that the country could still "be the responsible member of the international community that it claims to be rather than secretly trying to undermine it".
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