Experts believe hackers in Moscow are already trying to smash their way into vital computer networks that could bring the UK to a standstill.
Key targets include airports and rail links, hospitals, water, electricity and gas supplies and banks. Sources said that critical facilities including the national grid and the NHS have been placed on high alert.
Spy chiefs also believe that Kremlin backed hackers could release embarrassing information on ministers, MPs and other high-profile figures which have either been fabricated or been illegally obtained. It is feared that the operation could be combined with a “fake news” campaign.
Britain’s cyber-intelligence agency GCHQ and the Ministry of Defence are said to be on standby to retaliate “proportionately”.
It comes after Vladimir Putin failed to carry out threats to shoot down missiles fired at Syria in airstrikes on the country’s chemical weapons stockpile.
Britain’s relations with Russia have sunk to an historic low after the nerve agent attack on former Russian spy Sergei Skripal and his daughter Yulia in Salisbury last month which prompted mass expulsions of diplomats.
Yesterday Foreign Secretary Boris Johnson said of the threat: “We have to take every possible precaution and when you look at what Russia has done, not just in this country but the attacks on TV stations, on the democratic processes, on the critical national infrastructure, of course we have to be very, very cautious indeed.”
Cyber attacks could lead to electricity supplies being cut, air traffic control systems compromised and could lead to thousands of hospital operations being cancelled.
Counter-terrorism expert Professor Michael Clarke said: “I suspect Russia will choose not to respond in military terms. But cyber warfare is highly likely.
“A Russian attack in the next two to three weeks is a high threat. It will be an attack on national infrastructure, not just upsetting City firms, but getting inside the transport system, or the health system, or air traffic control. It could affect everyone.”
The alert comes just days after intelligence chiefs issued an unprecedented warning of the threat.
GCHQ chief Jeremy Fleming said of Russia: “They’re not playing to the same rules.
“They’re blurring the boundaries between criminal and state activity”.
Ciaran Martin, director of the National Cyber Security Centre revealed the threat had been ramped up to its highest level.
He said: “Cyber attack capabilities are an integral part of Russia’s national policy, of its way of asserting itself in the world.
“The NCSC will be on high alert and trying to do everything in its power to frustrate or prevent an attack.”
Last month military intelligence chief General Sir Chris Deverell warned Russia had developed the capacity to cripple Britain adding there was “no limit” to Putin’s methods of attack.
He said: “What they seek to do is to steal, plant, manipulate, distort, destroy our information.
“Every single system we have in our lives is in some way controlled by systems that have ones and noughts in them. If you can get them then you can affect us.”