Russia is "weaponising misinformation" in an effort to destabilise the West, Defence Secretary Sir Michael Fallon has warned.
In a no-holds-barred speech, Sir Michael accused Moscow of using cyber tools to "disrupt critical infrastructure and disable democratic machinery" in a series of attacks on western countries.
He said that it was vital that Nato strengthened its cyber defences while doing more to combat "false reality" being propagated by the Kremlin.
His warning came after new US president Donald Trump said he wanted to establish better relations with Russian president Vladimir Putin.
Speaking at St Andrews University, Sir Michael reiterated the prime minister's warning on her US visit last week, that when it came to Putin, they should "engage but beware".
Sir Michael pointed to a "persistent pattern of behaviour", highlighting a series of attacks which had been linked to Russia, including:
The taking off air of France's TV5Monde television station in April 2015
Targeting Germany's lower house of parliament later the same year which shut down its network
A cyber attack on Bulgaria in October 2016 described by the country's president as "the most heaviest and intense" to be conducted in south eastern Europe
Hacking the two main political parties in the US presidential elections
In addition, he highlighted that people are warning of Kremlin interference in upcoming European elections in Germany, Montenegro, and a Dutch referendum on an EU-Ukraine treaty.
He said these actions could not be dismissed as "business as usual", and that Putin has chosen to become a "strategic competitor" of the West.
"Today we see a country that in weaponising misinformation has created what we might now see as the post-truth age," he told the audience.
"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 said more should be done to "call out" messengers spreading "Soviet-style misinformation", such as Kremlin-backed RT television station and Sputnik news agency.
He also backed President Trump's call for all Nato member states to honour the committment to spend 2% of GDP on defence, while bolstering cyber capabilities, pointing to the UK almost doubling its investment in cyber capabilities to £1.9 billion.
"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 said.