Downing Street suggested Mr Putin’s actions so far – which include sending troops into parts of eastern Ukraine that Russia has recognised as independent states – did not yet amount to a “full-scale invasion”.
Russia’s ambassador in the UK, Andrei Kelin, was summoned to the Foreign Office in an official show of British condemnation for Moscow’s actions.
The Prime Minister said on Tuesday that the Russian president had “completely torn up international law” and is seemingly intent on capturing the capital of Kyiv.
Mr Johnson’s response came as allies sought to punish Moscow after the Kremlin ordered troops into two breakaway regions of eastern Ukraine, allegedly to carry out “peacekeeping” duties.
German Chancellor Olaf Scholz took the significant step of blocking the certification of the Nord Stream 2 pipeline that would have supplied gas directly from Russia to Germany.
The dramatic escalation came after Mr Putin recognised the Donetsk and Luhansk regions in the Donbas as independent states.
After chairing an early morning emergency meeting of the Cobra committee, Mr Johnson said he would later reveal sanctions against entities in Russia and the Donbas.
“This is, I should stress, just the first barrage of UK economic sanctions against Russia because we expect, I’m afraid, that there is more Russian irrational behaviour to come,” he told broadcasters.
“I’m afraid all the evidence is that President Putin is indeed bent on a full-scale invasion of the Ukraine, the overrunning, the subjugation of an independent, sovereign European country, and I think, let’s be absolutely clear, that will be absolutely catastrophic.”
In a sign the situation could deteriorate even further, Russia said its recognition of independence for the areas in east Ukraine also extends to territories currently held by the forces of the Kyiv government.
Mr Johnson said that if Mr Putin continues down on the path to “encircling Kyiv itself, which is what he seems to be proposing to do, capturing the Ukrainian capital” then it is vital his efforts “should not succeed and that Putin should fail”.