Boris Johnson warned that “gloomy” intelligence suggested Russia was planning a lightning raid on Kyiv as British staff and their families began leaving the Ukrainian capital.
The Prime Minister warned President Vladimir Putin that an invasion of Ukraine would be a “disastrous step” which could see Russia bogged down in a bloody and protracted conflict.
Mr Johnson said he did not believe war was inevitable and there was a chance that “sense can still prevail”.
Invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business
But confirming the exit of some British staff from the embassy he said: “We do think it prudent to make some changes now.
“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.”
He warned that the people of Ukraine would resist any invasion and “from a Russian perspective, (it) is going to be a painful, violent and bloody business”, he said.
“I think it’s very important that people in Russia understand that this could be a new Chechnya.”
The United States has also ordered the families of all American personnel at the US Embassy to leave the country in response the the risk of an invasion.
But the European Union’s foreign policy chief Josep Borrell said: “We are not going to do the same thing” and Ukraine’s foreign ministry spokesman Oleg Nikolenko said the US decision was “a premature step” and a sign of “excessive caution”.
Mr Johnson indicated he would have talks with allies, including US President Joe Biden, later on Monday.
The Prime Minister’s comments came after Mr Biden suggested that a “minor incursion” may result in a more measured response by the United States and allies.
Mr Johnson said “at any invasion, any incursion, of any kind, of any dimension, into Ukraine is not going to be a cost-free business… there will be casualties”.
Downing Street said British combat troops would not be used to defend Ukraine.
The Prime Minister’s official spokesman said around 30 British diplomats, including the ambassador, remained in Ukraine.
But he said there were “no plans to send UK combat troops” to the region but sanctions were on the table if Russia proceeded.
The spokesman said: “We don’t speculate on things like sanctions, but certainly there will be significant economic measures put in place.”
Russian forces have massed at the border with Ukraine and intense diplomatic activity has failed to ease tensions.
The Foreign Office said: “Some embassy staff and dependants are being withdrawn from Kyiv in response to the growing threat from Russia.
“The British Embassy remains open and will continue to carry out essential work.”
Meanwhile, Nato committed more ships and fighter jets to eastern Europe.
Denmark is sending a frigate to the Baltic Sea and is set to deploy F-16 fighter jets to Lithuania, Spain is sending ships and is considering sending fighter jets to Bulgaria, France has expressed its readiness to send troops to Romania, and the Netherlands is sending two F-35 fighter planes to Bulgaria from April.