Boris Johnson has warned Russian president Vladimir Putin that invading Ukraine would backfire and only serve to strengthen Nato.
The Prime Minister said he was considering dispatching RAF Typhoon fighters and Royal Navy warships to protect south-eastern Europe after sending 350 British troops to Poland.
Mr Johnson, writing in The Times, said the UK “will not flinch now” and that Britain will remain “unconditional and immovable” in supporting the Nato defence alliance.
With Russia having amassed an estimated 130,000 trips near its border with Ukraine, Foreign Secretary Liz Truss will head to Moscow for talks as diplomatic attempts to avert a war continue.
Mr Putin has claimed that the West is using Nato to undermine Russia.
But Mr Johnson wrote: “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.
“So I hope the Kremlin might yet realise how its objectives would not be served by inflicting still greater destruction and bloodshed on Ukraine.”
Earlier in the day, Defence Secretary Ben Wallace said the 350 British troops being dispatched to Poland will send a strong signal that the two countries “stand side by side”.
Poland is also facing a crisis on its own border with Belarus, whose leader Alexander Lukashenko is an ally of Russian leader Vladimir Putin.
Mr Wallace said the extra troops will add to 100 Royal Engineers already in Poland.
He denied that Nato is trying to use “divide and rule” tactics against Moscow.
“Nato is a defensive alliance. It poses no threat to Russia. It is a self-defence mechanism amongst our allies. That is what it is there for. No one wants to divide and rule Russia,” he said.
The Russian Government's aggression against Ukraine 🇺🇦 is part of a decades-long campaign to destabilise European security.— Foreign, Commonwealth & Development Office (@FCDOGovUK) February 6, 2022
The UK 🇬🇧 will defend freedom & democracy wherever it is threatened, and that is why we and our allies #StandWithUkraine. pic.twitter.com/O1WQSAmLaS
Ms Truss will travel to Moscow later this week for discussions with counterpart Sergei Lavrov.