Russia's invasion of Ukraine has now begun as Vladimir Putin ordered troops into two breakaway regions, the UK has declared.
UK Health Secretary Sajid Javid said everyone was "waking up to a very dark day in Europe" after President Putin formally recognised the Donetsk and Luhansk regions of Ukraine, where Moscow-backed separatists have revolted against Kyiv.
Boris Johnson chaired a dawn COBRA meeting on Tuesday as he prepared to slap a "barrage of sanctions" on Russia.
The Prime Minister said Mr Putin appeared to be "bent on a full scale invasion of the Ukraine" - and warned the result would be "catastrophic".
Mr Johnson is expected to give a statement on the new sanctions plan to MPs later today.
The Foreign Office has hauled in the Russian ambassador following Mr Putin's decision to send "peacekeeping" forces into the territory.
Mr Javid told Sky News: "We have seen that he has recognised these breakaway eastern regions in Ukraine and from the reports we can already tell that he has sent in tanks and troops.
"From that you can conclude that the invasion of Ukraine has begun."
He added: "We are waking up to a very dark day in Europe and it's clear from what we have already seen and found out today that the Russians, President Putin, has decided to attack the sovereignty of Ukraine and its territorial integrity."
The Top Tory compared the Ukraine situation to the Cuban Missile Crisis at the height of the Cold War, saying: "I do think it's as serious a situation as that."
Mr Javid said it was "not too late" for Mr Putin to step back from the brink but added: "We have to be realistic - clearly that does not seem his intent, certainly at this point in time, and we have to respond very significantly."
Individuals close to the Kremlin would be targeted by the new sanctions regime, he said.
"There will be sanctions, the Prime Minister has been clear about that," he told LBC.
"I am sure from what has already been shared by the Prime Minister that individuals that have close contacts with the Russian regime will be targeted, as will certain economic sectors."
After hosting the emergency Cobra meeting, the PM said: "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.
"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 the UK will work to ensure that "Putin should fail" if he carries out a full-scale invasion of Kyiv, "which is what he seems to be proposing to do".
He told broadcasters that the UK will continue diplomatic efforts to avert violence but added: "I just want everybody to be in no doubt that if Vladimir Putin continues down this track of violence, of aggression, of a full-scale invasion, of encircling Kyiv itself which is what he seems to be proposing to do, capturing the Ukrainian capital, it is absolutely vital that that effort, that conquest of another European country, should not succeed and that Putin should fail."
Downing Street said Russia's actions amount to "a renewed invasion" of Ukraine.
The PM's official spokesman said: "There is no doubt that the deployment of these forces that we've seen reported in sovereign Ukrainian territory amounts to renewed invasion of the country.
"President Putin has sent his troops in, he's broken international law, he's repudiated the Minsk agreement.
"We believe that Russia's actions overnight could well be a precursor to a full-scale invasion.
"But clearly, we want to continue to pursue any diplomatic avenues that exist and we'll be speaking to other world leaders, as I say, and urging Russia to step back from this action."
Foreign Secretary Liz Truss urged all Brits to leave Ukraine as soon as possible.
"The safety and security of British nationals in Ukraine is our top priority," she said.
"All Brits should leave now via commercial routes while they are still available.
"We are bolstering our teams in the region to support British people as they leave and once they have crossed the border."
Mr Johnson spoke to Ukrainian President Volodymyr Zelenskyy on Monday night to express his concerns, and said he would consider further "defensive support".
"Outlining his grave concern at recent developments in the region, the Prime Minister told President Zelensky that he believed an invasion was a real possibility in the coming hours and days," Downing Street said.
The Kremlin claimed Russian forces will "maintain peace" in eastern Ukraine.
