Around 8,000 British soldiers are due to be sent to eastern Europe to take part in military exercises designed to combat Russian aggression.
The deployment – which is one of the largest to take place since the Cold War – will see troops placed in countries from Finland to North Macedonia.
Dozens of tanks and more than 100 armoured fighting vehicles will also be sent to the locations this summer under plans that have been bolstered since the war in Ukraine began.
Live updates on the war in Ukraine
The armed forces will be joined by tens of thousands of troops from NATO and Joint Expeditionary Force allies, which include the likes of Denmark, Iceland and Norway.
The Ministry of Defence (MoD) said the planned exercises will improve the ability of UK and Finnish troops to work alongside each other, “deterring Russian aggression in Scandinavia and the Baltic states”.
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Defence Secretary Ben Wallace said the move will show “solidarity and strength”.
“The security of Europe has never been more important. These exercises will see our troops join forces with allies and partners across NATO and the Joint Expeditionary Force in a show of solidarity and strength in one of the largest shared deployments since the Cold War,” he said.
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“Operating across Europe, the British Army will stand alongside partners, combining our capabilities and shared values, promoting peace and security.”
The UK deployment is expected to build to a peak of around 8,000 personnel operating in mainland Europe between April and June.
Some of the exercises are set to involve parachute drops, helicopter-borne air assaults Challenger 2 tanks.
The drills aim to “showcase the scale and significance of the British Army’s contribution to the defence of Europe”, the MoD added.
Commander Field Army Lieutenant General Ralph Wooddisse explained: “The UK makes a significant contribution to the defence of Europe and the deterrence of Russian aggression. The British Army’s series of exercises is fundamental to both.
“The scale of the deployment, coupled with the professionalism, training and agility of the British Army, will deter aggression at a scale not seen in Europe this century.”
Exercises alongside American troops are also taking place in Poland.
Western powers are attempting to show their support for Ukraine is not waning
The Ministry of Defence says this is one of the largest military deployments since the Cold War and they are blunt that this is about showing solidarity and strength – no prizes for guessing who the intended audience is.
These exercises are to deter Russian aggression, to deter Vladimir Putin from entering a new conflict.
We understand that the military exercises the UK is involved in will peak between now and June, with 8,000 British army troops involved.
But if you count all of the troops from all of the NATO and the Joint Expeditionary Force countries together, then we are talking about tens of thousands of armed forces personnel.
Of course this comes as Joe Biden has asked Congress for $33bn (£27bn) for military, economic, and humanitarian assistance for Ukraine.
Western powers here are attempting to show their support is not waning as the conflict enters its third month.
Ukrainian capital comes under attack after meeting between Zelenskyy and UN secretary-general
Meanwhile, the Ukrainian capital of Kyiv has continued to come under fire, with at least one person killed and several others injured during a strike on Thursday night.
Rescue officials said some civilians were trapped beneath rubble after the Russian attack, which came shortly after a meeting between Ukraine’s President Volodymyr Zelenskyy and UN Secretary-General Antonio Guterres.
Mayor Vitali Klitschko said the Shevchenkivskyi district, in the northwestern part of the city, was hit twice, causing fires in at least two high-rise buildings.
Explosions have also been reported in Chernihiv, which is near the border with Belarus, and Fastiv, in the central part of the country.
Ukrainian authorities also reported intense Russian fire in the Donbas – the eastern industrial heartland that has become a key target for Russia.
Earlier on Thursday, a Ukrainian military official said Russian forces were “exerting intense fire” as they continued the second phase of their invasion.
Former British soldier killed in Ukraine as second Briton reported missing
It comes after a British military veteran was reported to have been killed in Ukraine while fighting Russian forces.
Tributes have been paid to 36-year-old Scott Sibley, who is the first British national confirmed to have died in the war.
A second British man, who Sky News has chosen not to name, has been reported as missing in Ukraine after travelling there to fight.
At the beginning of the war, Ukraine’s government urged volunteers from across the world to come and help support its armed forces as they took on Russia’s invasion.