Boris Johnson will travel to Europe this week for a last-ditch effort to avert war as a top Tory warned missiles and bombs could hit Ukrainian cities "minutes" after Russia gives the order.
The Prime Minister will hold talks with world leaders before travelling to Europe in the coming days as Downing Street warned an invasion could come "at any moment".
Tensions are escalating over the presence of an estimated 130,000 Russian troops on the border of Ukraine, prompting warnings from the UK and Nato allies to citizens to flee the country.
Armed Forces Minister James Heappey warned Europe was closer to war than at any point over the past 70 years and urged British nationals to leave Ukraine.
Parliament could be recalled from recess to show the UK's resolve in opposing Russia, he suggested.
Mr Heappey told Sky News: "It's impossible to know how many have left so far because this is a very different type of country to Afghanistan.
"There is free movement across the border of Ukraine into the European Union and people can just get in their cars and drive across into Poland."
He added: "This isn't a warning about something that could happen in three months' time, this isn't a warning that will be followed by further warnings because greater imminence has been reached.
"This is a warning because minutes after Putin gives the order, missiles and bombs could be landing on Ukrainian cities, and that means British citizens should leave now whilst they have the opportunities to do so."
Labour leader Keir Starmer said Parliament should be recalled if Russia invades Ukraine.
Speaking on a visit to Sunderland, he said: "I think it's very important that Parliament is recalled if there is an invasion.
"I'm sure the Government would do that anyway, because we must have a swift and strong response, a united response, from the United Kingdom and a united response with our allies."
Mr Johnson is receiving daily intelligence briefings on the crisis ahead of a trip to Europe later this week.
A Downing Street spokeswoman said: "The crisis on Ukraine's border has reached a critical juncture. All the information we have suggests Russia could be planning an invasion of Ukraine at any moment.
"This would have disastrous consequences for both Ukraine and Russia.
"There is still a window of opportunity for de-escalation and diplomacy, and the Prime Minister will continue to work tirelessly alongside our allies to get Russia to step back from the brink."
No 10 did not give further details of his travel plans, but it is understood that he plans to engage with Nordic and Baltic countries.
Foreign Secretary Liz Truss is also expected to visit Europe, while Defence Secretary Ben Wallace will head to Brussels for a meeting of Nato defence ministers this week.
Moscow denies it is planning an invasion and Russian Foreign Ministry spokeswoman Maria Zakharova accused the White House of stoking "hysteria" but US intelligence suggests the Kremlin could fabricate a "false flag" pretext to attack.
It comes as Ukraine's ambassador to the UK insisted the country still had ambitions to join Nato - after appearing to suggest it would drop its bid.
Vadym Prystaiko told the BBC that Ukraine would consider "serious concessions" including removing the goal of joining the Nato alliance - which has angered Russia.
But asked whether Ukraine might reconsider its Nato ambitions, he later said: "No this is not and I am quite happy that I have this chance to clarify my position."
Defence Secretary Ben Wallace faced criticism over the weekend after telling the Sunday Times that there is a "whiff of Munich in the air", in an apparent reference to failed diplomacy in the run up to the Second World War.
But a source close to Mr Wallace explained that his frustrations centred on if Mr Putin strikes "come what may, then all the diplomacy would have been a straw man", rather than being aimed at any European allies.
Mr Johnson is expected to make a series of visits to push his domestic agenda this week as he battles to regain control after the damaging Partygate row.
He will make a trip to Scotland on Monday and is also expected to travel to the North West to visit an oncology centre as part of efforts to slash the NHS backlog.
