A thousand British troops are on stand-by to provide humanitarian support to Ukraine if Russia invades.
The soldiers are in state of readiness ahead of a visit by Prime Minister Boris Johnson to the leaders of NATO and Poland on Thursday.
They could be deployed to Eastern Europe if Vladimir Putin orders his troop into Ukraine, in a move that might trigger a refugee crisis.
Johnson will go to Brussels and Warsaw to stress the need to hold firm on NATO's principles, and discuss ways that Britain can provide military support while Russia amasses its troops near Ukraine's border.
The Prime Minister's trip is one among a wave of international diplomacy.
French President Emmanuel Macron met Putin earlier this week, while US Vice President Kamala Harris is due to have in-person meetings with US allies at the Munich Security Conference next week.
Britain's foreign secretary and defence secretary are also due in Moscow this week for talks with their counterparts.
"The UK remains unwavering in our commitment to European security," Johnson said in a statement.
"As an alliance we must draw lines in the snow and be clear there are principles upon which we will not compromise."
On Monday the UK government said it would send a further 350 troops to Poland, after it deployed 100 there last year to help with a migrant crisis at its border with Belarus.
Johnson's office repeated on Wednesday that any further military incursion of Ukraine by Russia would likely create the mass forced displacement of people on Europe's border, affecting countries like Poland and Lithuania.
On Tuesday the PM said that he is considering sending RAF Typhoon fighters and Royal Navy warships to the region.
Johnson wrote in The Times : "If he launches another invasion, he will force the West to bring about much of what he seeks to prevent. In fact this is already happening.
"Because of his build-up, America, France, Italy and other allies are deploying forces to NATO's south-eastern flank, just as the UK reinforces the northeast.
"Nor could there be a more compelling argument for the necessity of NATO than the sight of Russian tanks invading a European country once again."
With Russia having amassed an estimated 130,000 trips near its border with Ukraine, Foreign Secretary Liz Truss is to head to Moscow for talks as diplomatic attempts to avert a war continue.
