British nationals have been urged by the Foreign Office to leave Ukraine immediately 'while commercial means are still available'.
The warning has been issued over concerns of a possible invasion by Russian forces.
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.
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“We urge British nationals in Ukraine to leave now via commercial means while they remain available.”
Defence Secretary Ben Wallace has said that Russia could mount an invasion of Ukraine 'at any time' and warned that conflict would have 'tragic consequences' for both countries.
Following talks in Moscow with his Russian counterpart Sergei Shoigu, Mr Wallace said he had received an assurance the Kremlin was not planning to attack its southern neighbour.
Mr Wallace told a news conference in the British embassy: "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.
“It is beyond normal exercising therefore we will judge that statement on the evidence.”
His news conference took place as Boris Johnson joined other Western leaders on a conference call organised by US President Joe Biden to discuss the situation in the region.
Also taking part were French President Emmanuel Macron, German Chancellor Olaf Scholz and Nato secretary-general Jens Stoltenberg, as well as EU leaders Ursula von der Leyen and Charles Michel.
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'.
He admitted that 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', adding: "I was clear about the tragic consequences that any invasion of Ukraine could have for all people – both Ukrainian (and) Russian – and the security of Europe.
"I think we have had a constructive and frank discussion and I hope it has contributed to a better atmosphere but also to de-escalation, but there is still considerable way to go between the two of us.”
He also explained that 2,000 anti-tank missile launchers supplied by the UK to the Ukrainian military were purely defensive.
Rebecca Shepherd