London (CNN) - Britain is bracing for the possibility of Russian retaliation in the wake of Saturday's airstrikes on alleged chemical weapons sites in Syria.
Foreign Secretary Boris Johnson said Britain must "take every possible precaution" against any potential Russian counter-measures and acknowledged that recent relations between Moscow and London had "not been good."
"When you look at what Russia has done, not just in this country, in Salisbury, attacks on TV stations, on the democratic processes, on critical national infrastructure -- of course we have to be very, very cautious indeed," Johnson said on the BBC's Andrew Marr Show on Sunday, when asked about the possibility that Russian could launch cyber-attacks on the National Health Service or electricity grid.
The UK's National Cyber Security Center said Monday that the threat of a cyber-attack against Britain is now "at its highest possible level."
Britain joined the United States and France in striking several targets in Syria on Saturday in retaliation for a suspected chemical attack on April 7 near Damascus that left dozens dead.
Russia, a key ally of Syrian President Bashar al-Assad, has warned of "grave repercussions" for the operation, which it described as an "illegal action."
Russia has been accused of carrying out cyber-attacks several times in recent years. When the Trump administration announced sanctions in March related to Russian meddling in the 2016 US election, it also disclosed what it described as a Russian attempt to penetrate the US energy grid.
Russia has denied allegations of election meddling and has denied accusations of other cyber-attacks on power grids in the past.
Saturday's airstrikes have further aggravated relations between Britain and Russia, which were already at their lowest in decades following a nerve agent attack on a former Russian double agent and his daughter in England last month.
The UK government blamed Russia for the poisoning, and the episode prompted the expulsions of Russian diplomats from more than 20 countries. Russia, which denied involvement, responded with its own diplomat expulsions.
"Contacts with the Russians have not been good," Johnson admitted on the Andrew Marr Show. He described Russian allegations of British involvement in the alleged chemical attack in Syria and the poisoning in Salisbury as "preposterous" and "demented."
Chemical watchdog blocked from Douma?
As the diplomatic fallout of Saturday's airstrikes continued, the UK on Monday accused Russia and Syria of blocking experts from the Organization for the Prohibition of Chemical Weapons (OPCW) from accessing Douma, the site of the suspected gas attack.
The OPCW had been due to carry out an investigation of the area, where dozens of people reportedly died and an estimated 500 people were injured.
But the UK envoy to the OPCW said in a tweet Monday that "Russia and Syria have not yet allowed access to Douma. Unfettered access essential. Russia and Syria must cooperate."
Russian Deputy Foreign Minister Sergei Ryabkov said that experts had instead been delayed from arriving in Douma due to the US-led airstrikes over the weekend, according to state news agency RIA Novosti.