Armed Forces minister James Heappey said British troops helping with training in Ukraine will be leaving the country this weekend.
Having sent UK personnel to train Ukrainians on the anti-tank missiles supplied by Britain, Mr Heappey said: "All of them will be withdrawn. There will be no British troops in Ukraine if there is to be a conflict there."
He added to BBC Radio 4's Today programme: "They will be leaving over the course of the weekend."
Former chief aide to No 10 Dominic Cummings has warned that Boris Johnson will be occupied by responding to police questions on alleged COVID-19 breaches.
But Mr Heappey said: "I suspect that like me the Prime Minister is reading the intelligence products he’s getting from SIS and GCHQ rather than reading Dom Cummings' tweets."
He also said Britons must leave Ukraine immediately because Russia is now at a stage where it can attack "at no notice", although Russia has denied planning to invade Ukraine.
Mr Heappey told BBC Breakfast: "We are now confident that the artillery systems, the missile systems and the combat air are all in place that would allow Russia to launch – at no notice – an attack on Ukraine.
"And on that basis I think it is our responsibility to share with UK citizens our view that they should leave the country immediately while commercial means are still available.
"There will be a big difference between what they may have seen on their TV screens in Afghanistan over the summer and what may happen over the next week or so, and that is that the Royal Air Force will not be in a position to go in and to fly people out so they need to leave now by commercial means or drive out of Ukraine into a neighbouring country."
Tory MP Tom Tugendhat said the Ukrainian military is "increasingly capable to defend themselves" and advised that Britain training them up is better assistance than sending troops.
The chairman of the Commons Foreign Affairs Committee told BBC Radio 4's Today programme: "We're enabling them to have the ability to fight themselves, and having served in combat in countries around the world I can tell you that training local forces to fight for themselves is a significantly better defensive technique than putting troops in.
"The reality is that the Ukrainians already have some 145,000 in their army, they have another – depending on how you count – 100-odd thousand border guard reserves and people like that so they have a significantly larger army even than we do and they are increasingly capable to defend themselves."
Sir Malcolm Rifkind described suggestions that there should be a NATO division inside Ukraine as "very unwise".
He told Times Radio: "There can't be a NATO division in Ukraine; Ukraine's not a member of NATO and you cannot send troops to that country without being involved in what could turn out to be a full-scale war. That is not going to happen".
Asked about earlier comments by Tobias Ellwood, the chairman of the Defence Select Committee, who called for such a division, Sir Malcolm said: "If he means a combat division, I think that's very, very unwise.
"If he's talking about people who might help train some Ukrainians, that’s happened before in various countries around the world."
Mr Ellwood called for British-led NATO divisions to be in the country and said: "An invasion is imminent. Once that happens, because of the grain the comes out of Ukraine for the world, (that will) affect food prices across the world.
"Oil and gas prices will be affected as well, and European security will then be threatened further, so we have to ask ourselves, what should we do instead?
"What are the calculations, and yes, there is this looking Putin in the eye wondering what would happen. This is our Cuban missile crisis moment".
He said the consequences of allowing Ukraine to fall would see a "new era of instability with a Russia and China axis developing" while the West is "shrinking in size" and authoritarianism is on the rise.
Sir Malcolm Rifkind also said NATO had taken "a very important decision" in providing defensive equipment for Ukraine, enabling Ukrainians "to have a better chance when they do fight back" and sending a message to Russian President Vladimir Putin "that the cost of invasion will be very strong".