The decision came during NATO crunch talks between world leaders amid escalating tensions with Russia. France’s Emmanuel Macron, Germany’s Olaf Scholz, Poland’s Andrzej Duda and Italy’s Mario Draghi were also on the call, along with the leader of NATO. A Downing Street spokesperson said: “The leaders agreed on the importance of international unity in the face of growing Russian hostility. The Prime Minister underlined the real human cost of any Russian aggression and the need to take all steps within our power to prevent that outcome.
“The leaders stressed that diplomatic discussions with Russia remain the first priority, and welcomed talks that have already taken place between Russia and Nato allies.
“They also agreed that the international community should not tolerate any action which undermines Ukrainian sovereignty. The Prime Minister outlined the steps the UK has taken to increase Ukraine’s defensive capacity. He emphasised the need to support Ukraine’s defences against the full spectrum of malign Russian activity.
“The leaders agreed that, should a further Russian incursion into Ukraine happen, allies must enact swift retributive responses including an unprecedented package of sanctions. They resolved to continue co-ordinating closely on any such response.”
This comes as Russia has stationed around 100,000 troops on its border with Ukraine.
In response, 8,500 US troops have been put on heightened alert for possible deployment to eastern Europe, the Pentagon has said.
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Mr Johnson said: “The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see.
“We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step.”
He said that “from a Russian perspective”, any invasion is “going to be a painful, violent and bloody business”.
He added: “I think it’s very important that people in Russia understand that this could be a new Chechnya.”
“I’ve got to tell you that I think the intelligence is pretty gloomy at this point.
“There is certainly a very, very large array of Russian forces and we have to take the necessary steps.
“I don’t think it’s by any means inevitable now, I think that sense can still prevail.”
Speaking about the people of Ukraine, the Prime Minister said: “My judgement is that they will fight.”
Downing Street confirmed that there are “no plans to send UK combat troops” to Ukraine.
Meanwhile, the Foreign Office said it was withdrawing some embassy staff from the country’s capital.
A spokesperson said: “Some embassy staff and dependants are being withdrawn from Kiev in response to the growing threat from Russia.
“The British Embassy remains open and will continue to carry out essential work.”
The Prime Minister said around 30 British diplomats, including the ambassador, remained in the country.