Britain could soon be at war with Russia after Vladimir Putin launched a devastating invasion of Ukraine, it has been warned.
Thousands of people have been seen trying to flee the Ukrainian capital of Kyiv this morning after Russia missile strikes began against the city.
Roads have been gridlocked as citizens try to escape the full-scale invasion.
Others have waited in long queues for buses or took shelter in subway stations and basements as they desperately prepare to flee the capital and Russia's airstrike assaults.
Speaking on BBC Radio 4's Today programme, General Sir Richard Shirreff, a retired Army officer, said any incursion into Nato territory would bring Britain into direct involvement in the conflict.
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"There is no surprise and there is profound sadness and a sense of appalling horror at what is about to unfold for the people of Ukraine," he said.
"And I think we have to assume that this is not Russia biting off a chunk of Ukraine - for example, establishing a land corridor in Crimea - but a full-blown military offensive to occupy Ukraine."
Aircraft flying to or from UK airports are being ordered to avoid Ukraine airspace after the crisis in the country intensified.
Wizz Air cancelled its flights between Luton Airport and the Ukrainian cities of Kyiv and Lviv on Thursday.
Ryanair and Ukraine International Airlines – the other carriers which fly between the UK and Ukraine – are also expected to suspend those routes.
The lack of flights will make it harder for British nationals to evacuate from Ukraine to the UK.
In Ukraine, horrifying photos have shown Ukrainian firefighters arrive to rescue civilians after an airstrike hit an apartment complex in Chuhuiv, Kharkiv Oblast.
One harrowing photo showed a man mourning a body while in tears with debris strewn from the airstrike.
It's also been reported that a Ukrainian border guard has died as a result of a rocket shelling from occupied Crimea.
State Border Guard Service said Russia had fired arms on them from occupied Crimea onto Preobrazhenka village, Kherson Oblast.
Ukraine Defence Minister Oleksiy Reznikov has called on all Ukrainians who are able to hold a weapon to mobilise, and people only need a passport to join their ranks.
Prime Minister Boris Johnson is chairing emergency Cobra talks and is set to reveal more sanctions against Russia when he addresses MPs later.
He's already said the UK will respond decisively to Russia's "unprovoked attack" on Ukraine and will make an address to the nation this morning.
The PM has also confirmed he will speak to the other G7 leaders and is calling for an urgent meeting of all Nato leaders as soon as possible.
And Sir Richard said it is "entirely plausible" that Russian President Vladimir Putin could be aiming to revive the Soviet Union, adding that if Russia puts "one bootstep" into Nato territory, the entire alliance will be at war.
Asked whether Britain might be expected to take part directly in the current military confrontation, he said: "Absolutely there is a possibility that we as a nation could be at war with Russia, because if Russia puts one bootstep across Nato territory, we are all at war with Russia. Every single one, every single member of the Nato alliance.
"Article 5 (of the Nato alliance) says an attack on one is an attack on all, so we need to change our mindset fundamentally, and that is why I say our defence starts in the UK on the frontiers of Nato."
