Whitehall departments have been issued guidance over possible Russian cyber-attacks on key British infrastructure.
Officials from the National Cyber Security Centre have also warned the National Grid, major power companies, the Sellafield nuclear power plant and NHS hospitals to prepare for possible action from Vladimir Putin's government, including cyber-attacks that cause a blackout or theft of data.
The NSCS, a branch of intelligence agency GCHQ, has issued advice to key organisations on boosting security and improving cyber-defences, according to reports.
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It comes after the Kremlin announced tit-for-tat expulsions of British diplomats in response to prime minister Theresa May's decision to kick out 23 Russian embassy staff. The British Council, the UK body for overseas cultural relations, will also be expelled from Russia.
The head of the NCSC, Ciaran Martin, warned in January of the risk of a 'category one' Russian cyber attack on the UK, which would involve "disruption of critical systems".
“They’re contacting all the critical national infrastructure operators. They’ve been in touch with National Grid with guidance," a Whitehall source told the Sunday Times.
The NCSC's director of operations, Paul Chichester, told the same paper: “It is absolutely right that we give advice to sectors on defending themselves from cyber-attacks.
“We are vigilant to cyber-threats wherever they come from and are ready to defend against them.”
Foreign secretary Boris Johnson will address EU foreign ministers tomorrow as they plot a joint response to last week's nerve agent attack in Salisbury, which the UK has pinned on the Russian state.
The prime minister will also hold a meeting of her National Security Council on Tuesday.
Among possible further sanctions are an emergency law to seize the assets of Russians laundering money through the UK, targeting London oligarchs' finances and strengthening the visa regime to stop associates of the Putin government coming to this country.