Britons told to leave Ukraine immediately amid threat of invasion by Russia

Britons have been told to flee Ukraine immediately over growing concerns that Russia could launch an invasion in the coming days.

The Foreign Office updated its advice on Friday evening to urge UK nationals to "leave now while commercial means are still available".

The order was issued as intelligence and advice from experts on the ground suggested an increased threat level, with an invasion at some point deemed likely, the PA news agency understood.

• RAF fighters welcome US B-52 bombers back to Europe as Russia tensions build
• More UK troops go on standby amid Russian invasion fears
• Ukraine invasion will only strengthen NATO, Prime Minister warns Putin

It came after Boris Johnson voiced fears for the security of Europe during a call with western leaders including US President Joe Biden.

Defence Secretary Ben Wallace said an invasion could come "at any time", with Russian President Vladimir Putin having amassed an estimated 130,000 troops on the border with Ukraine.

Mr Biden’s national security adviser Jake Sullivan said an attack before the end of the Winter Olympics on February 20 was a "credible prospect", assigning it a "very, very distinct possibility".

He said new Russian forces were arriving at the border and are in a position to "mount a major military operation in Ukraine any day now", which could include a "rapid assault on the city of Kyiv" or on other parts of the country.

Speaking from the White House, Mr Sullivan said Russia could choose "in very short order to commence a major military action against Ukraine", but stressed the US does not know whether Mr Putin has made a final decision.

Mr Sullivan said the "threat is now immediate enough" to urge Americans to leave Ukraine "as soon as possible and in any event in the next 24 to 48 hours".

In a call lasting around 80 minutes, Downing Street said Mr Johnson urged NATO allies to make it clear to Moscow there is a "heavy package of economic sanctions ready to go".

Watch: NATO holds its breath over possible Russian invasion of Ukraine.

"The Prime Minister told the group that he feared for the security of Europe in the current circumstances," a No 10 spokeswoman said, in an account of the call that included 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.

Mr Johnson warned that the penalties would be "extremely damaging" to Russia's economy and urged that allies must reinforce NATO's eastern frontiers.

The Foreign Office followed the US in advising against all travel to Ukraine, with a spokesman saying: "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."

Earlier in the day, 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.

"It is beyond normal exercising therefore we will judge that statement on the evidence."