Russia has launched a full-scale invasion of Ukraine after months of escalating tensions, sparking condemnation around the world as decades of peace in Europe came to an end.
Ukrainian forces have battled invaders around nearly all of the country's perimeter throughout Thursday after Moscow mounted a mass assault by land, sea and air.
The attack marks the biggest attack on a European state since the Second World War.
Nations across the world have condemned Russia's invasion with Boris Johnson vowing that Britain "cannot and will not just look away".
The PM pledged to "hobble the Russian economy" with sanctions and late on Thursday unveiled the government’s largest ever package of economic sanctions against Russia, including an asset freeze on major Russian banks.
Nato secretary-general Jens Stoltenberg told a briefing: "This is a deliberate, cold-blooded and long-planned invasion.
"It is a blatant violation of international law, an act of aggression against a sovereign, independent and peaceful country and a serious threat to Euro-Atlantic security."
Watch: Nato chief condemns Russia's invasion of Ukraine in ‘strongest possible terms’
Senior Conservative MP David Davis said that Putin has "effectively declared war on the West," and called for the UK to provide air support to Ukraine.
Will Britain go to war with Russia?
Not at the moment, no, and it would be a considerable escalation if the UK was to enter a military conflict with Russia.
This does not mean it will not happen. On Thursday, General Sir Richard Shirreff said the UK could soon be at war with Russia.
Ukraine is a friend of Nato, but it is not a member, and so the UK and its allies are not required to defend it.
Watch: Russia invades Ukraine: Video shows missile strike on Ukrainian air base, Sky News verifies
Article 5 of Nato's founding treaty states that any attack on a member should be considered an attack on all, and all members are required to defend each other.
Russia does share a border with Nato states, namely Estonia and Latvia, as well as with Poland and Lithuania via the Kaliningrad exclave.
Although Putin has never directly threatened Estonia, Latvia and Lithuania (collectively known as the Baltic States) he has spoken frequently about his regret that so many nations were able to break away from Russia when the Soviet Union broke up.
Belarus, which analysts have called a Russian puppet state, also shares a border with Lithuania and Poland.
If any Russian military action is taken against the Baltics or Poland then the UK (as well as all other Nato states) would be at war with Russia.
What is Britain doing now?
The UK has responded to the invasion by imposing sanctions in an attempt to cripple Russia's economy.
The first tranche were announced earlier in the week, before Putin launched a full-scale attack, targeting Russian banks and three high net-worth individuals.
Boris Johnson on Thursday revealed a broader package of sanctions on over 100 Russian individuals and entities.
"Overall we will be imposing asset freezes on more than 100 new entities and individuals on top of the hundreds that we've already announced," the PM told the House of Commons.
"This includes all the major manufacturers that support Putin's war machine. Furthermore, we are also banning Aeroflot from the UK."
He added that Britain will cut off Russian banks from sterling markets and clearing payments, with the United States taking similar action on the dollar.
"These powers will enable us totally to exclude Russian banks from the UK financial system, which is of course by far the largest in Europe, stopping them from accessing sterling and clearing payments through the UK," the PM said.
Johnson is also rallying European leaders to sever its dependence on Russian gas that has given Vladimir Putin a “grip” on Western politics.
A number of Nato countries, including Germany, rely heavily on Russia for their power.
In addition, the UK already has a significant military presence in the Baltic States under a Nato mission, and has indicated it will send more troops there in response to the attack on Ukraine.
During an emergency Nato meeting, Bulgaria, the Czech Republic, Estonia, Latvia, Lithuania, Poland, Romania and Slovakia triggered urgent consultations under Article 4 of Nato’s founding Washington Treaty.
These are launched when “the territorial integrity, political independence or security of any of the (Nato) parties is threatened".
It is only the sixth time the article has ever been invoked.
Lithuania has also declared a state of emergency in a decree signed by president Gitanas Nauseda in response to Russia’s attack.
Speaking on BBC Radio 4’s Today programme, the retired Sir Richard said any incursion into Nato territory would bring Britain into direct involvement in the conflict.
"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.
Sir Richard said it is “entirely plausible” that 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.