Boris Johnson says "the shock will echo around the world" if Russia invades Ukraine, which, he says, can still be avoided through diplomacy.
Plans for an invasion appear to be "in motion", he added, after warning that a Russian attack would bring about the "destruction of a democratic state".
The Prime Minister said that aggression in separatist-held areas in the east of Ukraine had the potential be a "prelude to bigger action", with the West fearing a so-called "false flag" operation that could give Moscow cover to wage war on Kyiv.
The Prime Minister is in Germany, where he is urging allies to show solidarity to avoid a "catastrophic act of aggression".
He told world leaders at the Munich Security Conference that he believed there is enough collective resolve among the UK's European allies to ensure that any Russian invasion would fail.
Mr Johnson added: "Every time that western ministers have visited Kyiv, we have assured the people of Ukraine and their leaders that we stand full square behind their sovereignty and independence.
"How hollow, how meaningless, how insulting those words would seem if, at the very moment when their sovereignty and independence is imperilled, we simply look away.
"If Ukraine is invaded, the shock will echo around the world."
Watch: US Secretary of Defense says Putin will see 'a stronger NATO on his flank'.
Mr Johnson said diplomacy could still prevent the "disaster" of a war in eastern Europe, if the West speaks with "one voice", speaking before he arrived in Bavaria.
Fears grow that instability in Russian separatist-held areas of Ukraine could spark an invasion by Moscow forces.
A separatist leader in eastern Ukraine has ordered a full military mobilisation during a spike in violence in war-torn Donetsk, including the shelling of a humanitarian convoy.
The West is concerned it could be used as a pretext for invasion by Russian President Vladimir Putin.
American President Joe Biden warned on Friday that the US has reason to believe Russian forces "intend to attack" Ukraine in the coming days, including targeting capital Kyiv.
Mr Biden told a White House press briefing on Friday he was "convinced" Mr Putin had "made the decision" to move his military across the border, having spent weeks saying he thought the Russian leader was undecided.
US Vice President Kamala Harris told the Munich Security Conference that US forces would not deploy to fight alongside Ukraine but would protect every inch of NATO territory.
Boris Johnson is later due to meet with counterparts to discuss tensions on Ukraine's border – it’s reported 150,000 Russian troops are amassed.
Upon his arrival in Munich, he was greeted by British ambassador Jill Gallard.
In a video that appeared to have been recorded during the flight to the summit, the Prime Minister said: "I'll be urging unity in the face of potential Russian aggression in Ukraine and that unity is absolutely vital if we're going to deter what I think would be an absolutely catastrophic act of aggression by Vladimir Putin.
"My message today is that there is still time to avert that disaster, that diplomacy can prevail, and that's the message I'll be taking to Munich."