Armed Forces Minister James Heappey said Russia could invade Ukraine with a deadly attack without any notice. Some 130,000 Russian soldiers have been massed on the border.
On Friday, the Foreign Office advised British citizens in Ukraine to leave the country “while commercial means are still available”.
Mr Heappey told Sky News on Monday, “this isn’t a warning about something which could happen in three months time”.
“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 while they have the opportunity,” he said.
He added: "We've seen now that there's 130,000 combat troops around Ukraine's land borders. There's thousands more on amphibious shipping in the Black Sea.
"Russia has concentrated artillery missile systems and combat air in the area. And, perhaps more ominously, although it grabs less headlines, all of the combat enablers, the logistics, the fuel, the medical supplies, the bridging assets, are all also now in place.
"So, the reason that the travel advice changed on Friday and the urgency of the messaging since is that the attack could effectively now happen with no notice."
Britons with family in Ukraine have described their fear at being unable to see their loved ones or bring them to the UK as the prospect of a Russian invasion looms.
Gary Smith, 53, met his wife Helen, 55, in 2012 and has regularly flown to and from Ukraine to visit her.
Mr Smith said: “I’m here and she’s there, and I can’t do anything about it. That’s the frustrating part - the ball is in Putin’s court.”
Mr Smith’s wife is Russian but has lived in Ukraine for more than 30 years and now works as a university lecturer.
As the process to secure a visitor’s visa would take weeks, Mr Smith said she has no plans to leave at the moment.
“She’s got a life there now, her life is in Kharkiv,” Mr Smith added.
“If the Russians do invade - God forbid they do - it’ll be a long time before I see Helen.”
Jez Myers, a business consultant who has split his time between Manchester and Kyiv since 2018, cannot return to the country as planned to see his Ukrainian partner.
“I was due to fly back tomorrow morning... but it would be naïve at best to not follow the FCDO’s advice,” the 44-year-old said.
“I’m worried for her safety.”
Mr Myers expressed concern over the lack of a clear plan for him and his girlfriend.
“There’s just a lack of clear information,” he said.
“For a Brit, you should leave the country - and if you have a Ukrainian partner, they should apply for a visa.
“But my partner, she’s faced with this situation (and thinking): ‘Well this is my country, this