Boris Johnson has urged Vladimir Putin to “step back from the brink” as fears grow that Russia is on the verge of invading Ukraine.
Downing Street said the prime minister will “work tirelessly” this week with international leaders as part of last-ditch effort to avoid war.
Johnson is receiving daily intelligence briefings on the situation in the region, with security chiefs fearing a Russian invasions could happen “at any moment”.
The PM will is expected to travel to Europe towards the end of the week, and an announcement is expected on further economic and military support for Ukraine.
In a sign of the worsening security situation in Ukraine, British nationals have been advised to leave the country immediately while commercial flights remain available.
A Downing Street spokesperson said: “The crisis on Ukraine’s border has reached a critical juncture. All the information we have suggests Russia could be a planning an invasion of Ukraine at any moment. This would have disastrous consequences for both Ukraine and Russia.
“There is still a window of opportunity for de-escalation and diplomacy, and the Prime Minister will continue to work tirelessly alongside our allies to get Russia to step back from the brink.”
Defence Secretary Ben Wallace will attend a meeting of NATO defence ministers in Brussels to work on the alliance’s response to the current tensions.
He announced yesterday that he was cancelling a family holiday in response to the “worsening” situation in Ukraine.
An estimated 130,000 Russian troops are massed along Ukraine’s border but the Kremlin has insisted it is not planning an attack.