British Prime Minister Boris Johnson warned Russian President Vladimir Putin not to attack Ukraine.
Johnson has threatened to deploy fighter jets and warships to Eastern Europe to protect Kiev's NATO neighbors.
Ukraine is not a member of NATO even though it has volunteered to be a member of the alliance. Russia has warned that Kiev's joining the alliance would spark a war with Moscow.
PM Johnson said Britain "will not flinch now" and will stand by its fellow NATO countries in the event of an attack by Russia, which has amassed troops and weapons near the border with Ukraine.
He said he was considering sending Typhoon fighter jets and a British Royal Navy warship to Eastern Europe after sending 350 troops to Poland.
While he is also ready to ask Parliament to sanction Russian individuals and companies in case of an invasion.
With Russia amassing about 130,000 troops near its border with Ukraine, British Foreign Secretary Liz Truss will fly to Moscow for talks in a diplomatic bid to prevent war.
US officials say an attack by Russia in Ukraine could occur within days or weeks. However, Moscow denies that it is planning an invasion.
Putin has also claimed that the West is using NATO to undermine Russia.
PM Johnson wrote in The Times: "If he launches another invasion, he will force the West to do a lot of the things he is trying to prevent. Actually, this is already happening."
"Because of its troop buildup, America, France, Italy, and other allies are deploying troops to the southeast flank of NATO, just as Britain is strengthening the northeast," Johnson continued.
"Nor is there a more convincing argument about the need for NATO than to see Russian tanks attack a European country once again," he added.
"So I hope that the Kremlin may not have realized how its objectives will not be achieved by causing more destruction and bloodshed in Ukraine."
Earlier on Monday British Defense Secretary Ben Wallace said the 350 troops sent to Poland would send a strong signal that the two countries are "side by side".
Poland is also facing a crisis on its own border with Belarus, whose leader Alexander Lukashenko is an ally of Putin. Wallace said the additional troops would add to the 100 Royal Engineers already in Poland.
He denied that NATO was trying to use "divide and rule" tactics against Moscow. "NATO is a defense alliance. It poses no threat to Russia.
This is a self-defense mechanism among our allies. That's what they are for. Nobody wants to divide and rule Russia," he said.
Johnson said sanctions would be ready to be imposed if there was a Russian attack on Ukraine.
"UK sanctions and other measures will be ready for any renewed Russian strike," Johnson wrote.
"The government will ask Parliament for new powers to impose broader sanctions on Russian individuals and entities, including any companies linked to the Russian state or operating in strategic sectors important to the Kremlin."
Britain, Johnson added, is also preparing to strengthen the British-led NATO combat group in Estonia.
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