Russia is carrying out a sustained campaign of cyber attacks targeting democracy and critical infrastructure in the West, UK Defence Secretary Sir Michael Fallon has warned.
Moscow hoped to destabilise governments, expand its influence and weaken Nato by "weaponising misinformation" he said.
Sir Michael said Russian President Vladimir Putin had chosen to become a "strategic competitor" of the West.
Moscow said the claims were unfounded.
It came as UK Prime Minister Theresa May was preparing to press fellow Nato members to spend more on defence at an informal EU summit in Malta.
Meanwhile MPs have warned that a skills shortage and "chaotic" handling of personal data breaches are undermining confidence in the UK government's ability to protect its own infrastructure and economy from cyber attacks.
Sir Michael said Nato needed to do more to tackle a "false reality" being propagated by the Kremlin and said its member states must strengthen their cyber defences.
"Nato must defend itself as effectively in the cyber sphere as it does in the air, on land, and at sea, so adversaries know there is a price to pay if they use cyber weapons," he added.
The defence secretary pointed to a "persistent pattern of behaviour" by Moscow, highlighting a series of cyber attacks that had been linked to Russia.
Another cyber attack, on Bulgaria in October 2016, was described by the country's president as the "heaviest" and most "intense" to be conducted in south-eastern Europe.
Sir Michael also spoke about the suspected Russian hacking of the two main political parties in the US presidential elections.
Russia has denied any involvement in the hacking, and WikiLeaks founder Julian Assange has denied that Moscow was the source of Democratic Party emails leaked on his website. .
Sir Michael said: "Today, we see a country that in weaponising misinformation has created what we might now see as the post-truth age.
"Russia is clearly testing Nato and the West. It is seeking to expand its sphere of influence, destabilise countries and weaken the alliance.
"It is undermining national security for many allies and the international rules-based system.
"Therefore it is in our interest and Europe's to keep Nato strong and to deter and dissuade Russia from this course."
Sir Michael backed US president Donald Trump's call for all Nato member states to honour the commitment to spend a minimum of 2% of GDP on defence.
A Kremlin spokeman said: "We're very disappointed that minister Fallon is so aggressively disposed, we are sure that such an accusation against our country is unfounded.
"We don't see how a minister that has such a senior post can carry out such unfounded accusations that are based on nothing at all.
"We hope that in time, Great Britain will in some way take a constructive approach that will lead to a normalisation in relations between our two countries."