British nationals are warned to leave Ukraine now while commercial means are still available.
It comes as the US said that Russia could invade “any day now” and told its own citizens to leave immediately.
Moscow has denied intending to invade Ukraine, however, it has over 100,000 troops near the border.
British nationals are urged to keep their departure plans “under constant review” and ensure their travel documents are up to date.
The Foreign Office warned in a statement: “British nationals should not expect consular support or help with evacuating in the event of a Russian military incursion.”
Germany, Australia, Ireland, Italy, Sweden, Jordan, The Netherlands, Kuwait and Iraq have joined the UK and the US in instructing their citizens to leave the country.
A number of other countries are also advising to avoid travelling to Ukraine.
The Prime Minister said to other world leaders in a virtual meeting held on Friday he “fears for the security of Europe.”
He said if Russia make the “devastating and destructive decision” to invade, NATO need to have “a heavy package of economic sanctions ready to go.”
Johnson added President Putin must understand there would be “severe penalties” that would be “extremely damaging” to Russia’s economy.
The update in the travel advice is in response to Russia now being at a stage where an invasion could be imminent.
Speaking on BBC Breakfast, armed forced minister, James Heappey, said: “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.”
The Foreign Office estimates a few thousand British nationals remain in Ukraine.
A Foreign, Commonwealth and Development Office spokesperson said the safety and security of British nationals is their “top priority.”
As the threat of attack grows more serious, more embassy staff are expected to leave Kyiv. The British embassy would remain open, but in-person consular assistance would no longer be available.
The UK defence secretary Ben Wallace has cancelled a holiday and will be returning amidst fears of the invasion.
Wallace said to Sunday Times there is a “whiff of Munich in the air.”
It is believed to be a reference to the agreement that enabled the German annexation of Sudetenland in 1938 but failed to prevent the Second World War.
Ukraine’s ambassador to the UK, Vadym Prystaiko, criticised Wallace’s comment.
He told BBC Radio 4: “It’s not the best time for us to offend our partner in the world, reminding them of this act which not brought peace but the opposite – it brought war.”
The US State Department’s Derek Chollet warned there would be “crippling” economic sanctions on Russia if the attack went ahead.
Chollet said: “Whatever form it takes if it is as large as we fear it will be, the circumstances will be profound for Ukraine and quite dangerous.”