Britain's relationship with Russia is the worst it has ever been, Moscow's top diplomat in the UK has warned.
Ambassador Alexander Yakovenko last night attacked No 10 for "raising tensions in Europe" by deploying 800 troops to Estonia -part of Nato's eastern flank, according to Daily Mail.
The Kremlin's man in London said ministers were guilty of hostility, and that ties between the two countries had plummeted to an all-time low.
In an escalation of Moscow's rhetoric, he said there was no longer any "bilateral relationship of substance".
He added that the UK's actions recently had been "provocative" and even "outright ridiculous".
Yakovenko's remarks, which strike a more pessimistic tone than he has before, will deeply concern British officials.
But a diplomatic source said: "Russia must show it wants to work towards international goals and peace in Syria if there is to be any substantial improvement in our relations."
The comments came amid fears the world is on the brink of a nuclear conflict as Donald Trump, with UK backing, adopts a tougher stance towards adversaries.
Britain's relationship with Russia has deteriorated since the US President's revenge bombing on a Syrian regime air base, following a gas attack on civilians.
Boris Johnson cancelled his trip to Russia then said on Sunday that Vladimir Putin was in a "league of supervillains" because of his support for President Bashar al-Assad.
Last week Theresa May accused Moscow of being on the "wrong side of this argument" by failing to condemn a chemical attack in Syria. And Defence Secretary Sir Michael Fallon said Russia was responsible "by proxy" for the deaths of more than 80 civilians gassed by the regime.
Then on Saturday the Royal Navy escorted two Russian warships as they passed through the English Channel.
Last month hundreds of British troops, tanks and armour were sent to Estonia as part of the biggest show of force against Moscow since the Cold War.
Speaking for the first time since the recent heightened tensions, Yakovenko, a 62-year-old career diplomat who was handed the London role in 2011, said: "We deplore that deployment for it raises tensions in Europe along the border between Nato and Russia.
"Russia doesn't pose any threat to Estonia nor any other Nato member-state. That's why all the talk of territorial defence sounds provocative, and with [the] changing nature of war, outright ridiculous."
Asked if this is the worst that relations between the UK and Russia have ever been, he said: "Yes, it is." Previously officials have gone only as far as saying relations are the worst since the Cold War.
Yakovenko said he had "respect" for the Prime Minister, but when it came to her view that Russia was on the wrong side, "the opposite is true".
"There is no moderate opposition alternative to the present government in Syria," he said, adding that Syrians must "decide for themselves" who should run the country.
The UK Government has taken an unwavering stance on President Assad. Sir Michael has repeatedly said the dictator, who has killed his own people, must go.
Responding to comments that Russia was responsible for the deaths of Syrians by "proxy", Mr Yakovenko said "we deplore such hostile rhetoric".
He denied claims the regime was responsible for the chemical attack earlier this month, saying it looked like President Assad had been 'framed' by terrorists.
The diplomat compared this to an attack in Damascus in 2013.
At the time David Cameron said it was "beyond doubt" the Syrian regime was to blame, and called for MPs to back a military response.
Yakovenko said: "Now the same stagecraft with the same purpose in mind. It is suspicious that the West wouldn't even discuss the issue of chemical weapons use by opposition terrorists."
Foreign Secretary Johnson was accused of being a "poodle" for cancelling a diplomatic trip to Moscow following the Syrian strikes, leaving talks to US Secretary of State Rex Tillerson.
Tillerson was said to have issued an ultimatum, telling Moscow to side with America or stand alongside Assad and militants.
But Yakovenko said: "Fortunately, it turned out to be a mere suggestion ... But in history the very word 'ultimatum' is closely associated with a declaration of war."
He said Johnson's decision to cancel was "unfortunate" but "another indication that there is no bilateral relationship of substance between our two nations beyond mere diplomatic ones".
Writing in the Sunday Telegraph yesterday, Johnson said: "They [Russia] still have time to be on the right side of the argument."