Prime Minister Boris Johnson has warned Russia that invading Ukraine would be "disastrous" and a "painful, violent and bloody business".
Speaking as the Foreign Office pulled some embassy staff out of Ukraine, the PM said the situation was "pretty gloomy" but war was not inevitable.
He said the UK was "leading on creating a package of economic sanctions" against Russia and was supplying defensive weaponry to Ukraine.
Nato is putting forces on standby.
It said Nato allies were sending additional ships and fighter jets to eastern Europe to reinforce defences and increase deterrence, in response to the continuing build-up of Russian forces.
Russia has denied plans for military action, but an estimated 100,000 troops have amassed on the border. The head of Nato has warned of a risk of fresh conflict in Europe.
The Kremlin has accused Nato of escalating tensions with the move. Russia considers the alliance as a threat, and is demanding legal guarantees that it will not expand further east, including into Ukraine. But the US has said the issue at stake is Russian aggression, not Nato expansion.
Mr Johnson said: "The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see.
"We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step."
Mr Johnson added that the UK stood "four-square with the people of Ukraine".
US President Joe Biden held a video conference with EU leaders, the Nato secretary general and Mr Johnson on Monday evening.
Downing Street said the leaders agreed that if a further Russian incursion took place, "allies must enact swift retributive responses including an unprecedented package of sanctions".
Earlier, No 10 said there were no plans to send British combat troops to defend Ukraine, however.
Officials said there had been no specific threats to British diplomats but about half of the staff working in Kyiv will return to the UK. The US has ordered relatives of its embassy staff to leave, saying an invasion could come "at any time".
The embassy moves were described as precautionary, and nothing specific is thought to have occurred in the past 24 hours to have triggered the decisions of the US and UK.
About 30 British diplomats - including the ambassador - remain in Ukraine.
Foreign Secretary Liz Truss said the British Embassy in Kyiv continued to operate but "it's important that we prepare for any eventuality".
She said the UK had a "very strong package of sanctions ready to go" and called for Germany's Nord Stream 2 gas pipeline to Russia to be cancelled in the event of an incursion into Ukraine, saying she was "very, very concerned" about Europe's energy dependence on Russia.
Asked about the UK's accusations that Russian President Vladimir Putin was plotting to install a pro-Moscow figure to lead Ukraine's government, Ms Truss declined to elaborate on the source of the intelligence.
But she said: "We are going to call out every instance of Russia trying to influence democracy, trying to subvert Ukraine, false flag operations and sabotage. We will call that out."
Tensions over Ukraine
EU staff in Ukraine will stay in place for now - EU foreign policy chief Josep Borrell said he would not "dramatize" the tensions.
The prime minister said he had visited Ukraine and knew the people of the country, adding: "My judgment is that they will fight."
Mr Johnson, speaking to reporters in Milton Keynes, said: "We also need to get a message that invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business.
"I think it's very important that people in Russia understand that this could be a new Chechnya."
There was a major separatist conflict in Chechnya in the 1990s, with a decade of ultimately unsuccessful fighting for its independence.
Asked if an invasion was imminent, Mr Johnson said: "I've got to tell you that I think the intelligence is pretty gloomy at this point. There is certainly a very, very large array of Russian forces and we have to take the necessary steps.
"I don't think it's by any means inevitable now, I think that sense can still prevail."
He was due to speak to international allies on Monday.
Russia has seized Ukrainian territory before, when it annexed Crimea in 2014, following fierce protests in Ukraine that toppled the country's pro-Russian president. Russian forces seized control of Crimea before the territory voted to join Russia in a referendum the West and Ukraine deemed illegal.
Russian-backed rebels control areas of eastern Ukraine near Russia's borders in a conflict which has cost an estimated 14,000 lives. A 2015 peace deal is far from being fulfilled.
On Sunday, the UK Foreign Office accused Mr Putin of planning to install a pro-Moscow figure to lead Ukraine's government, naming former Ukrainian MP Yevhen Murayev as a possible candidate.
Mr Murayev called the claims "stupid" in an interview with Reuters news agency.
The Foreign Office has warned against all travel to Crimea and two areas of eastern Ukraine, Donetsk oblast and Luhansk oblast. It is advising against all but essential travel to the rest of Ukraine.
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