Boris Johnson has called the situation in Ukraine “very, very dangerous” and urged Vladimir Putin to step back from “the edge of a precipice”.
The Prime Minister has called on the Russian President to stand down as around 130,000 troops and heavy fire power are amassed at the Russian border with Ukraine.
Foreign Secretary Liz Truss chaired an emergency COBR meeting on Monday afternoon, while the PM traveled back to London after cutting short a trip to Cumbria.
Mr Johnson is expected to lead a fresh COBR meeting on Tuesday.
He told reporters the “evidence is pretty clear” that Russia is planning an invasion and that there were “all sorts of other signs that show that there are serious preparations” going on, but there was "still time" for Mr Putin to step back.
On a visit to Scotland, Mr Johnson said: “This is a very, very dangerous, difficult situation, we are on the edge of a precipice but there is still time for President (Vladimir) Putin to step back.”
He called for more dialogue and urged Russia to avoid a “disastrous” invasion.
Warning Mr Putin needed to understand the economic and political consequences if he launched an invasion of Ukraine, the PM said Europe needed to cuts its reliance on Russian hydrocarbons – including the Nord Stream pipelines.
“What I think all European countries need to do now is get Nord Stream out of the bloodstream," he said.
“Yank out that that hypodermic drip feed of Russian hydrocarbons that is keeping so many European economies going.
“We need to find alternative sources of energy … and get ready to impose some very, very severe economic consequences on Russia.”
Meanwhile, the Prime Minister's official spokesman said: "There is a grave possibility of an invasion this week.
"There remains a window of opportunity to deescalate and pursue a diplomatic path and the PM has been clear we must stand with our allies and combine deterrents with dialogue to deescalate this crisis."
Downing Street suggested Parliament could be recalled to in the event of an invasion.
The spokesman said: “In that situation you would expect the Prime Minister would want Parliament to be updated and to have its say."
The PM said his Defence Secretary Ben Wallace was “right to say” that the West must be united against Russian aggression.
Asked if he agreed with Mr Wallace’s claims there was a “whiff of Munich” and appeasement in actions against President Putin so far, Boris Johnson said: “I think the Defence Secretary is absolutely right to say it’s very important that we’ve got to be strong, and we’ve got to be resolute and we’ve got to be united.”
He also said: “I think it is very important that we all stand together and that we show a united front, particularly when it comes to economic sanctions.
“The UK, as you know, has been in the lead in bringing our allies together so that there’s a tough package of sanctions.
“It’s very, very important that the West should stand united, particularly that Nato has done it. It’s been pretty encouraging to see the way countries have pulled together.”
Asked if he would would be travelling to Moscow to negotiate with President Putin, Boris Johnson said he would be “doing everything I can to the diplomatic process”.
The Prime Minister said: “I’m going to be doing everything I can to help the diplomatic process. I’ll be talking to various leaders, including Joe Biden, very soon.”
He also said that Ukraine’s future membership of Nato could not be “bargained away” as he stressed the importance of continuing talks.
Mr Johnson told reporters: “I think it’s very important that we have a conversation, but what we can’t do is trade away the sovereign rights of the Ukrainians who aspire to Nato membership.
“That’s something that was a massive gain for our world. If you remember what happened in 1990, you had a Europe whole and free, countries could decide their own futures. We can’t bargain that away. It’s for the Ukrainian people.”
Ms Truss urged Britons to leave Ukraine because of the risk of an "imminent Russian invasion".
"We are fully aware that there could be a Russian invasion almost immediately," Ms Truss said after chairing crisis talks.
"That is why British citizens do need to leave Ukraine but we are also pursuing the path of diplomacy and de-escalation."
Ms Truss hit back at her Russian counterpart Sergei Lavrov after a frosty meeting last week when he described talking to her as "the deaf talking to the blind".
She said: "I went to Russia to deliver a very clear message, which is that it's Russia who is the aggressor, they have 100,000 troops on the Ukrainian border and if they stage an incursion into Ukraine, that would have a damaging effect on the Russian people and the Russian government.
"Of course, the Russians didn't like what I had to say but I say it nevertheless.
"And I want them to desist and I want them to be aware that there will be severe costs of an invasion."
