Britain blamed Russia on Thursday for a cyber-attack last year, publicly pointing the finger at Moscow for spreading a virus which swept across Europe and beyond, hitting companies and other users after initially targeting Ukraine.
Russia denied the accusation, saying it was part of "Russophobic" campaign that it said was being waged by some Western countries.
Two sources familiar with the matter said later on Thursday that London's announcement had been coordinated with the United States and other countries, some of which were expected to follow up with their own statements about who was responsible for NotPetya in the coming days.
The so-called NotPetya attack in June started in Ukraine where it crippled government and business computers before spreading around Europe and the world, halting operations at ports, factories and offices.
Britain's foreign ministry said in a statement the attack originated from the Russian military.
"The decision to publicly attribute this incident underlines the fact that the UK and its allies will not tolerate malicious cyber activity," the ministry said in a statement.
"The attack masqueraded as a criminal enterprise but its purpose was principally to disrupt," it said.
"Primary targets were Ukrainian financial, energy and government sectors. Its indiscriminate design caused it to spread further, affecting other European and Russian business."
"RUSSOPHOBIC CAMPAIGN"
Tariq Ahmad, a junior minister at the foreign ministry, said the attack cost European firms and organisations hundreds of millions of pounds.
"The Kremlin has positioned Russia in direct opposition to the West: it doesn’t have to be that way," he said. "We call upon Russia to be the responsible member of the international community it claims to be rather then secretly trying to undermine it."
Denmark's defence minister Claus Hjort Frederiksen was quoted as saying he also believed the cyber-attack originated in Russia. "The Russians have lifted this to a higher level where it’s just about causing damage like in a military attack,” Frederiksen told Ritzau news agency.
Moscow has previously denied being behind the NotPetya attack. On Thursday Kremlin spokesman Dmitry Peskov said Russia "categorically denies the allegations".
"We consider (them)... groundless. This is nothing more than a continuation of a Russophobic campaign that is without proof," Peskov told a conference call with reporters.
The Washington Post newspaper reported in January that the U.S. Central Intelligence Agency attributed the NotPetya attack to Russian military hackers.
British defence minister Gavin Williamson said on Thursday the attack was part of a new era of warfare and Britain had to be ready to respond. "We must be primed and ready to tackle these stark and intensifying threats," he said in a statement.
Britain has recently become more vocal about the threat posed by Russia at a time when some members of the ruling Conservative Party have expressed concern about the impact of cuts to defence spending.
In November, Prime Minister Theresa May accused Russia of meddling in elections and planting fake stories in the media.