Brits in Ukraine have been ordered to leave immediately as the whole country has been put on the red danger list.
The UK Foreign Office said in a statement on Friday evening the "safety and security" of its overseas nationals is a top priority, adding it had updated its travel advice accordingly.
The warning came after Boris Johnson voiced fears for the security of Europe during a call with world leaders including US President Joe Biden.
The eastern European country has appeared to be on the brink of an invasion from Russia for weeks.
More than 100,000 troops from the Kremlin have been moved to the border, though President Vladimir Putin has denied he plans to take the country.
An FCDO spokesperson said: "The safety and security of British nationals is our top priority, which is why we have updated our travel advice.
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"We urge British nationals in Ukraine to leave now via commercial means while they remain available."
A British man said he feels "worried" for the safety of his wife in Ukraine following Friday's announcement.
Gary Smith, 53, moved back to Newcastle for work in August after living in Ukraine for two years with his wife, who is a lecturer at a university there.
He said: "I am just hoping and praying that there isn't an invasion. If there is, a lot of lives will be lost.
"My wife said to me the other week 'I don't want to be killed.' What are you supposed to say to that?
"I moved back to the UK to make a life for us here and she was meant to join me within the next year.
"If this happens it could end up being three or four years before I can see her again."
Washington has warned that Russia was massing more troops near Ukraine and an invasion could come at any time.
While Mr Johnson told allies on Friday that he feared for the security of Europe due to the ongoing situation, his office said.
He held a virtual meeting with the leaders of the United States, Italy, Poland, Romania, France, Germany, the European Council, the European Commission and NATO to discuss the situation.
"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," his office said.
"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."
Yesterday, the US government also advised Americans against travelling to Ukraine due to "the increased threats of Russian military action".
The State Department also urged all those US citizens currently in Ukraine to leave the country now.
The FCDO's updated advice reads: "British nationals in Ukraine should leave now while commercial means are still available.
"Since January 2022, the build-up of Russian forces on Ukraine's borders has increased the threat of military action.
"Due to this increased threat, the FCDO has taken the decision to further withdraw embassy staff from Kyiv.
"The embassy remains open but will be unable to provide in-person consular assistance. British nationals should leave while commercial options remain."
It had previously advised Brits to travel to Ukraine for "essential" reasons, though border regions near Russia were marked as no-go areas.
