Boris Johnson has admitted that the threat of economic sanctions may not be enough to deter Russia from invading Crimea.
The prime minister warned that a Russian incursion into Ukraine would be a “bloody and protracted conflict” on a similar scale to the second world war.
Fears of an imminent invasion have been heightened in recent days following a further buildup of Russian troops on the border as well as rising tensions between government forces and Moscow-backed separatists backed in eastern Ukraine.
In a bleak assessment of the situation, the prime minister said Moscow’s plan for an invasion had “already in some senses begun”.
“That’s what our American friends think and you’re seeing these provocations now in Donbas — these explosions and so on — that we’ve been warning about for a long time,” he told the BBC’s Sunday Morning programme.
“The plan that we’re seeing is for something that could be the biggest war in Europe since 1945 just in terms of sheer scale.”
Moscow has continued to deny that it intends to invade and has accused the West of “hysteria”.
Western allies have attempted to offer its support to Ukraine by expressing solidarity through the military alliance Nato, and by offering armed forces training and anti-tank weapons.
But because Ukraine is not a Nato member, support is limited to providing weaponry and financial sanctions against Russia, rather than boots on the ground.
Johnson said penalties against associates of President Vladimir Putin, as well as firms of strategic importance to Russia, would hit the Kremlin “very hard”.
But he warned: “I’ve got to be absolutely frank with you, that may not be enough on its own.
“It will will be very damaging and very difficult...but it may not be enough to deter an irrational actor.
“And we have to accept at the moment that Vladimir Putin is possibly thinking illogically about this, and doesn’t see the the disaster ahead.”
Despite moves to bolster Ukraine’s defences against Russia in the event of an attack, the Western response has earned comparisons to the policy of appeasement as Europe was on the brink of war in the late 1930s.
Asked whether the message that was being sent to Ukrainians was that they were “on their own”, Johnson replied: “I don’t think that that’s fair.
“I think that what we’re trying to do is offer every possible support to Ukraine, and to make sure that we hit Russia with the hardest possible package of economic sanctions.
“What we’re also doing is fortifying the eastern flank of Nato.
“I think in Putin’s imagination, a lot of his anxiety is about about Nato and what he sees as the encouragement of Nato since the end of the Cold War.
“And I think what he wants to see is Nato push back and you can see the exact opposite.
“If he thinks he’s going to get less Nato as a result of this, he’s totally wrong — he’s going to get more Nato.”