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.

Boris Johnson is leading a crisis meeting of ministers and senior officials to consider the response to Vladimir Putin’s actions in Ukraine.

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.

The Prime Minister was scheduled to share calls with world leaders and will update MPs on the sanctions package at around 12.30pm.

“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.”

The map shows the parts Russia claims are independent.

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”.

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”.

Cabinet minister Sajid Javid said: “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.”

Foreign Secretary Liz Truss and Tim Barrow, Political Director at the Foreign, Commonwealth and Development Office, walk from 10 Downing Street, London, where the government's Cobra emergency committee has been meeting to discuss latest developments regarding Ukraine.

The Health Secretary 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.”

The Cobra meeting follows 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.

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“The humanitarian impact will be terrible on civilians fleeing the fighting. We know that women and children will suffer most.”

She said the Security Council must be united in calling on Russia to “de-escalate immediately”, as well as “condemning aggression against a sovereign nation and defending the territorial integrity of Ukraine”.

“Russia has brought us to the brink,” she said. “We urge Russia to step back.”

The Kremlin said Russian forces will “maintain peace” in eastern Ukraine.

Mr Johnson said the decision from Mr Putin to recognise Donetsk and Luhansk was an “ill omen” and a flagrant breach of international law.

The Cobra meeting will co-ordinate the UK response and agree a “significant package of sanctions to be introduced immediately”, No 10 said.

Following the meeting the Prime Minister will have a string of calls with other world leaders and will update MPs on the sanctions package at around 12.30pm.

Kurt Volker, a former US ambassador to Nato, said Russia’s actions in Ukraine have created a “very dire” situation.

He told BBC Radio 4’s Today programme that Russian leader Vladimir Putin’s decision to recognise two regions of Ukraine as independent states “opens the door to a lot of new steps by Russia”.

“We’ve already seen him putting forces into those territories. The portion of Luhansk and Donetsk that Russia occupies are not the entire territory of Luhansk and Donetsk.

“So that also opens the possibility that they will view Ukraine as an occupier of those territories, and then fight to remove Ukrainian forces and take more.

“That in turn could lead to much greater skirmishes and fighting with Ukrainian forces which could easily escalate out of control, so this is a very, very dire time.”

He said “devastating sanctions” should be imposed immediately.

General Sir Richard Sherriff, Britain’s former top Nato commander, has called the Ukraine situation the most perilous in Europe for decades.

He told BBC Radio 4’s Today programme: “This is the most dangerous moment in Europe probably at least since 1962 and the Cuban missile crisis, but it could be much worse than that because this is the invasion of a sovereign country which could turn into a catastrophic war with warfare on a scale not seen in Europe since 1945.

“So I think it’s difficult to overstate the seriousness of the situation.”

He said that while the war would initially be between Ukraine and Russia, the possibility that Nato could get involved is “very real”.

He told the programme: “Nato has got to man the ramparts now.”

He added that the West has to “assume the worst” when it comes to Russia’s nuclear strategy.

Ukraine conflict: Boris Johnson leads emergency meeting in response to crisis as Russa orders troops into Ukraine