Intelligence reports suggest Vladimir Putin’s plan to invade Ukraine has already begun, Downing Street said on Monday.
No10 added that “elements” of the Russian “playbook” for an attack were starting to “play out in real time”.
Ministers have highlighted what they believe to be a series of “false flag” operations, bombings and other strikes which the West believes Russia is seeking to wrongly blame on Ukrainian forces to justify a military intervention.
Cyber attacks have also been launched against Ukrainian banks, which the UK has blamed on Russian military intelligence.
The Kremlin has also been accused of a disinformation campaign, while Russian diplomats claim the West is pushing “fantasies” about an invasion.
Despite Russia having amassed up to 190,000 military personnel around Ukraine’s borders, Britain still believes that a diplomatic solution could be found to resolve the crisis, with the actual invasion yet to have been launched.
French president Emmanuel Macron raised hopes of an eleventh-hour summit between the Russian leader and US president Joe Biden to defuse the crisis.
However, the Kremlin said talk of such a high-stakes meeting was “premature”.
In London, the Prime Minister’s official spokesman said: “The intelligence we are seeing suggests Russia intends to launch an invasion and that President Putin’s plan has in effect already begun.
“We are seeing elements of the Russian playbook that we would expect to see in those certain situations starting to play out in real time.
“We have been working with Nato allies to call that out when we see it.”
He added: “Crucially, we still think there is a window for diplomacy.
“I think that is what we have seen with discussions over the weekend and we want to explore those.”
The latest development’s came hours after Mr Biden agreed “in principle” to meet the Russian premier.
But while Kremlin spokesman Dmitry Peskov said on Monday morning that Mr Putin and Mr Biden could meet if they consider it necessary, he stressed that “it’s premature to talk about specific plans for a summit.”
In a potentially worrying development, Moscow also announced plans for Mr Putin to address Russia’s National Security Council.
The UK and its western allies are pushing hard to force Mr Putin to the negotiating table.
French Foreign Minister Yves Le-Drian was due to talk by phone with Russian counterpart Sergei Lavrov later on Monday as Germany’s Foreign Minister Annalena Baerbock issued an urgent appeal to Russia to hold talks on resolving the crisis.
“I urgently call on the Russian government, on the Russian president: Don’t play with human lives,” Ms Baerbock said ahead of a meeting of EU foreign ministers in Brussels.
“What we have seen over the last 72 hours in terms of attacks, violent disputes is really concerning. The responsibility lies with the Russian government which is why I call urgently on the Russian government: Come back to the negotiating table. It is in your hands.”
But US intelligence officials reportedly believe Mr Putin may have already given orders to military commanders to begin final preparations for an attack.
Fighting in the Donbas region of eastern Ukraine, which has been the setting for an eight-year conflict between Ukrainian forces and Russian-backed separatists, was reported to be continuing on Monday with a blast reported in the rebel held city of Donetsk.
Officials in the separatist territories claim Ukrainian forces launched several artillery attacks over the weekend and that two civilians were killed during an unsuccessful assault on a village near the Russian border.
Western leaders have said they fear the Kremlin will use claims of such attacks as a justification for launching a full invasion.
Meanwhile Ukraine’s military said two soldiers died in firing from the separatist side on Saturday and reported a further 80 violations of ceasefire agreements in the region yesterday,
Adding to fears of an imminent invasion, Russia and its ally Belarus announced they were extending military drills on Belarusian territory, raising concerns that Mr Putin could launch an attack on the Ukrainian capital Kiev which is just 50 miles south of the Belarus border.
Business minister Paul Scully urged the Russian President to step back from a conflict, saying the “the loss of life will be horrendous”.
“There are clear signs of troop movements on the border, there’s 7,000 more troops than there were just a few days ago despite the Russians trying to signal they were pulling back from the border,” he told Sky News. “There’s a very credible threat.”