The defence secretary has warned it is "highly likely" that Vladimir Putin will order an attack on Ukraine, despite ongoing talks to avert a war.
Ben Wallace also said there is a "whiff of Munich in the air" - an apparent reference to the agreement that allowed the German annexation of the Sudetenland in 1938, but failed to prevent the Second World War.
Speaking to The Sunday Times, he warned that Russia has amassed 130,000 troops and heavy firepower along Ukraine's border, meaning Moscow could "launch an offensive at any time".
Mr Wallace added: "It may be that [Putin] just switches off his tanks and we all go home but there is a whiff of Munich in the air from some in the West."
And he warned that it was "worrying" that Russia's military build-up has continued despite high-level diplomatic talks increasing - prompting fears that the Kremlin is intent on invading Ukraine come what may.
Biden warns Putin in hour-long phone call
On Saturday, US President Joe Biden spoke to his Russian counterpart Vladimir Putin for over an hour - warning America and its allies will "respond decisively" if there is an invasion.
Talks have gained a sense of urgency after US intelligence suggested that the Kremlin could take action before the end of the Winter Olympics in Beijing on 20 February - far sooner than analysts had expected.
The Russian president told Mr Biden that Washington's response to Moscow's security demands had not taken into account key concerns, and the West had not put enough pressure on Ukraine to abide by the Minsk agreements.
A senior official in the White House described the call as professional and substantive, but that there was no fundamental change.
White House stoking 'hysteria,' Russia says
Maria Zakharova, a spokeswoman for Russia's Foreign Ministry, has accused Joe Biden's administration of stoking "hysteria".
Ukrainian President Volodymyr Zelenskyy also sought to downplay the threat, saying: "The best friend of our enemies is panic in our country. And all this information is just provoking panic and can't help us."
British nationals in Ukraine are being urged by the Foreign Office to "leave now while commercial means are still available" - with Armed Forces minister James Heappey warning that the RAF would not carry out evacuations if war broke out.
Israel, Portugal and Belgium have become the latest countries to order their citizens to leave, and Australia has begun evacuating its embassy in Kyiv.
Protests in Ukraine's capital
On Saturday, thousands of Ukrainians marched through the centre of Kyiv - chanting "Glory to Ukraine" and carrying banners that said "Ukrainians must resist" and "invaders must die".
Although Mr Zelenskyy has urged Ukrainians to remain calm, he agrees with Washington's assessment that a Russian attack could happen at any time, and attended police drills in the southern region of Kherson.
This has become the biggest security crisis between Russia and the West since the Cold War, and US officials believe they have a matter of days to prevent an invasion that could cause enormous bloodshed in the region.