Russia has continually denied responsibility for the attack, pointing out that Russian firms were also damaged by last summer's events.
Russian President Vladimir Putin's spokesman, Dmitry Peskov, rejected Britain's allegations as groundless, asserting that they were part of what he called a "Russophobic" campaign being conducted in some Western countries.
The destructive attack masqueraded as ransomware, but its goal was principally to disrupt.
An assessment by the National Cyber Security Centre has found that the Russian military was nearly certainly responsible for the "NotPetya" cyber attack of June 2017. Companies like the delivery service FedEx, container-ship giant Moeller-Maersk, pharmaceutical firm Merck, French construction firm Saint-Gobain, British advertising company WPP Group, and many others fell victims of the virus.
The UK's foreign office backed up Williamson's remarks, with minister Lord Ahmad saying in a statement (via Reuters): "The decision to publicly attribute this incident underlines the fact that the UK and its allies will not tolerate malicious cyber activity".
"We categorically deny the accusations".
Companies with apparent strong links with Ukraine suffered badly from the NotPetya attack.More news: Unbreakable Kimmy Schmidt season 4 sets premiere in two parts
More news: Drake takes 63-year-old hotel maid on $10000 shopping spree
More news: Liberals to change how Indigenous-rights cases are handled in courts
It is estimated to have cost companies more than $1.2 billion.
Foreign minister for cyber security Lord Ahmad of Wimbledon said that the cyber attack showed a "continued disregard for Ukrainian sovereignty".
"The decision to publicly attribute this incident reiterates the position of the United Kingdom and its allies that malicious cyber activity will not be tolerated", it added.
British army chief Nick Carter later said that Russian cyber-warfare presented a direct threat to Britain.
London has taken an aggressive stance against Moscow, with Prime Minister Theresa May a year ago accusing it of "seeking to weaponise information".
Some British politicians have accused Russian Federation of attempts to disrupt the democratic process in Britain by online interference in political campaigns such as the 2016 Brexit referendum and a 2017 general election.
"The Kremlin has positioned Russian Federation in direct opposition to the West yet it doesn't have to be that way", Lord Ahmad also said.
"There is still no effective response from the West either in the form of countermeasures or sanctions", he said.