Boris Johnson has called for an “overwhelming display of western solidarity” in the face of a potential Russian invasion of Ukraine.
The prime minister is due to deliver a speech at the Munich security conference on Saturday morning.
Western allies have accused Vladimir Putin of planning to fabricate a pretext to launch an attack.
US president Joe Biden has said there are no signs of a promised Russian withdrawal – but instead has seen more troops moving toward the border.
“Every indication we have is they’re prepared to go into Ukraine, attack Ukraine,” he said.
According to The Times, the British government also believes Putin has made up his mind to invade.
Speaking ahead of his trip to Munich, Johnson said: “There is still a chance to avoid unnecessary bloodshed, but it will require an overwhelming display of western solidarity beyond anything we have seen in recent history.
“Allies need to speak with one voice to stress to President Putin the high price he will pay for any further Russian invasion of Ukraine. Diplomacy can still prevail.
“That is the message I will take to Munich today as we redouble our efforts to prevent a grave miscalculation which would devastate Ukraine, Russia and the rest of Europe.”
Meanwhile Putin has accused Ukraine of “systematic” human rights violations against the country’s Russian-speaking population as Moscow ratcheted up the pressure in its stand-off with the West.
The Russian president insisted that large-scale military exercises with Belarusian forces close to the Ukrainian border were “purely defensive” and did not represent a threat to any other country.