The UK government has taken an unusual step in publicly pinning the blame for last year’s so-called NotPetya cyber attack on Russia.

Cyber experts at the UK’s National Cyber Security Centre said the GRU Russian military intelligence agency was almost certainly responsible for the ransomware NotPetya attack in June.

The attack is thought to have been targeted at Ukrainian financial, energy and government sectors but the malicious software spread other European and Russian business.

The malware is estimated to have cost companies more than $1.2bn with consumer goods giant Reckitt Benckister, Dutch delivery firm TNT and Norwegian shipping giant Maersk all affected.

Read more: What was the Petya attack?

British defence secretary Gavin Williamson told the BBC that Russia was “ripping up the rule book” when it came to international warfare.

Williamson in January told the Daily Telegraph a Russian attack on Britain’s national infrastructure could kill thousands and the country has been researching the UK’s critical national infrastructure and how it connects to continental power supplies with a view to creating “panic” and “chaos”.

Russia has denied responsibility for the NotPetya cyber attack, pointing to Russian firms that were among those whose systems were affected as evidence of its innocence.

Foreign Office minister Lord Ahmad said the UK’s decision to publicly accuse Russia of the attack “underlines the fact that the UK and its allies will not tolerate malicious cyber activity”.

The UK government judges that the Russian government, specifically the Russian military, was responsible for the destructive NotPetya cyber attack.

Its reckless release disrupted organisations across Europe costing hundreds of millions of pounds. The Kremlin has positioned Russia in direct opposition to the West yet it doesn’t have to be that way.

We call upon Russia to be the responsible member of the international community it claims to be rather then secretly trying to undermine it.

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The Kremlin has positioned Russia in direct opposition to the West, yet it doesn’t have to be that way. We call upon Russia to be the responsible member of the international community it claims to be rather then secretly trying to undermine it.

The NotPetya attack was similar to the WannaCry outbreak that took place in May last year, as the computer systems appear to be locked down with the ransomware around the world, and messages are appearing demanding $300 worth in bitcoin.

Over 300,000 computers were infected by the WannaCry malware, which also took down 46 NHS England Trusts.