Vladimir Putin appears "bent on a full-scale invasion" of Ukraine, Boris Johnson warned as he vowed to swiftly introduce the "first barrage" of sanctions against Russia.
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 warnings came after Moscow ordered troops into two breakaway regions of eastern Ukraine, allegedly to carry out "peacekeeping" duties.
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."
Mr Johnson said that if Mr Putin continues down 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".
He said he would set out economic sanctions that "will hit Russia very hard" and target the “economic interests that have been supporting Russia's war machine" in the Commons later in the day, warning that they will only worsen "in the event of an invasion".
Earlier, Health Secretary Sajid Javid told Sky News that from Moscow's actions "you can conclude that the invasion of Ukraine has begun".
The Cobra meeting followed an emergency session of the UN Security Council where the UK's ambassador, Dame Barbara Woodward, said Russia has "brought us to the brink", warning that the country's actions "will have severe and far-reaching consequences".
She said an invasion would unleash "the forces of war, death and destruction" on the people of Ukraine.
"The humanitarian impact will be terrible on civilians fleeing the fighting. We know that women and children will suffer most," she added.
The Kremlin insisted Russian forces will "maintain peace" in eastern Ukraine.
Russia has previously always denied planning to invade Ukraine, despite amassing an estimated 190,000 troops along the Ukrainian border.
Watch: UK 'must prepare' for the consequences of Russian invasion into Ukraine. Video thumbnail: Erik Romanenko/TASS/Alamy.
Mr Johnson was scheduled to share calls with world leaders and will update MPs on the sanctions package at around 12:30 UKT.
The Prime Minister told Ukrainian President Volodymyr Zelensky on Monday evening that he would "explore sending further defensive support to Ukraine", according to Downing Street.
Later, Mr Zelensky told his nation "we are not afraid of anyone" after Russia recognised the independence of the separatist regions.
The UK has been sending defensive military equipment to Ukraine amid the heightened tensions, including light anti-armour weapons with training from British troops and kit such as body armour and helmets.
The European Union and United States are also imposing sanctions in response to the crisis.