The Foreign Office on Friday urged all British nationals to leave Ukraine immediately as the US warned an invasion could take place “any day now”.
The Government issued its advice minutes before a White House press conference when US national security adviser Jake Sullivan said an assault could begin during the Winter Olympics.
It came as UK Defence Secretary Ben Wallace warned an invasion could come “at any time”, while Boris Johnson voiced fears for the security of Europe during a call with Western leaders.
Russian President Vladimir Putin has amassed an estimated 130,000 troops on the border with Ukraine - although he has repeatedly denied he is planning an attack.
But Mr Sullivan said “the Russians are in the position to be able to mount a major military operation in Ukraine any day now."
A Foreign Office spokesman said: "The safety and security of British nationals is our top priority, which is why we have updated our travel advice.
"We urge British nationals in Ukraine to leave now via commercial means while they remain available."
It also advised against all travel to Ukraine, in its new advice issued on Friday evening.
The number of Britons in Ukraine is understood to be in the low thousands.
Many of them are believed to live in the country, with some married to Ukrainians, or with business interests, and so some of them may remain.
More staff may be withdrawn from the British Embassy in Kiev though an operational capacity will remain. Dozens of commercial flights a day are taking place between Ukraine and the UK. People can also leave the war-threatened country by road, including by crossing into Poland.
The US had earlier warned Americans in Ukraine that they should leave as he would not send troops on a rescue mission if Russia invades. US President Joe Biden warned on Thursday night “things could go crazy quickly” as he urged American citizens to leave immediately.
On Friday evening UK time Mr Sullivan warned that an invasion could come before the end of the Winter Olympics. The Games, taking place in China, will finish in nine days time on February 20.
Speaking from the White House, he said: “We continue to see signs of Russian escalation including new forces arriving at the Ukrainian border.
“As we’ve said before, we are in the window of when an invasion begins at any time should Vladimir Putin decide to order it.
“I will not comment on the details of our intelligence information but I do want to be clear it could begin during the Olympics despite a lot of speculation that it will only happen after the Olympics.
“Any American in Ukraine should leave as soon as possible and in any event in the next 24 to 48 hours.”
Western leaders held a virtual meeting to discuss the crisis on Friday afternoon.
Mr Johnson told allies during the talks that he fears for the security of Europe due to the situation in Ukraine, his office said.
A Downing Street spokesperson said: “The Prime Minister told the group that he feared for the security of Europe in the current circumstances.
“He impressed the need for Nato Allies to make it absolutely clear that there will be a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine.
“The Prime Minister added that President Putin had to understand that there would be severe penalties that would be extremely damaging to Russia’s economy, and that Allies needed to continue with efforts to reinforce and support the Eastern frontiers of Nato.
“He urged the leaders to work together to deliver economic and defensive support to Ukraine.
“The leaders agreed that if President Putin deescalated, there was another way forward, and they pledged to redouble diplomatic efforts in the coming days.”
The meeting also included the leaders of the United States, Italy, Poland, Romania, France, Germany, the European Council, the European Commission and Nato.
Mr Wallace held talks in Moscow with Russian defence minister Sergei Shoigu as part of diplomatic efforts to avert war.
"Currently there's over 130,000 troops stationed at readiness or exercising - plus warplanes, plus ships into the Black Sea - on the borders of Ukraine and that is an action that is not normal," Mr Wallace told a news conference in the British embassy following the talks.
"It is beyond normal exercising therefore we will judge that statement on the evidence."
Following a frosty meeting in Moscow on Thursday between Foreign Secretary Liz Truss and her counterpart Sergei Lavrov, Mr Wallace said his discussions with Mr Shoigui had been “frank and constructive”.
While he said that he took the minister's assurances “seriously”, he admitted he was less optimistic than he had been previously that there could be a diplomatic solution to the crisis.
He said the current disposition of Russian forces meant they could do “a whole range of actions, including an invasion of a neighbouring country, at any time”.
The tensions over Ukraine have risen in recent months amid a row over demands include a ban on Ukraine entering Nato and a limit to the deployment of troops and weapons to Nato’s eastern flank.
The current tensions come eight years after Russia annexed Ukraine’s southern Crimea peninsula. Since then, Ukraine’s military has been locked in a war with Russian-backed rebels in eastern areas near Russia’s borders.
The crisis has escalated in recent months amid the troop build up and demands from Moscow of a ban on Ukraine entering Nato.
Mr Putin has also demanded a limit to the deployment of troops and weapons to Nato’s eastern flank. Nato has rejected the Russian demands.
The Pentagon said on Friday it is sending another 3,000 combat troops to Poland to join 1,700 who already are assembling there in a demonstration of American commitment to Nato allies worried at the prospect of Russia invading Ukraine.
Mr Johnson said this week that Nato’s efforts in the region, including the UK providing more troops to train Ukrainian soldiers and 2,000 anti-tank missiles, are to “avert what could be one of the worst crises on our European continent in decades”.
Nato nations including the US, the UK and Germany have warned of tough sanctions if Russia does move into Ukraine.
Ms Truss said last month a situation in which British soldiers would fight alongside Ukrainians against Russia is “very unlikely.”
She described how Britain has been offering support to Ukraine through intelligence, cyber support and defensive weapons.
