The prime minister, in a televised address to the nation, said the Russian president had "unleashed war in our European continent" with a "hideous and barbaric venture".

Russia launched a military offensive overnight against its neighbour with missile strikes and explosions reported across Ukraine.

Mr Johnson accused Mr Putin of having "attacked a friendly country without any provocation and without any credible excuses".

"Innumerable missiles and bombs have been raining down on an entirely innocent population," the PM added.

"A vast invasion is underway by land, by sea and by air."

Russian helicopters 'downed' - Ukraine invasion latest

Key developments at a glance:
• Explosions heard in major cities including Kyiv and Kharkiv
• Shelling in western city of Lviv reported by Ukrainian emergency service
• Ukraine says Russia has launched a "full-scale invasion" and "war of aggression"
• Vladimir Putin ordered a "military operation" in Ukraine's Donbas region
• Joe Biden says US and its allies will respond amid global condemnation
• EU drawing up 'unprecedented' sanctions

PM: 'We cannot and will not just look away'

As he pledged the UK's support for Ukraine, Mr Johnson appeared to dismiss any notion of 1930s-style appeasement in dealing with the crisis.

"This is not, in the infamous phrase, some 'faraway country of which we know little'," he said, a reference to a speech by Neville Chamberlain in 1938 about the German annexation of the Sudetenland.

"We have Ukrainian friends in this country; neighbours, co-workers. Ukraine is a country that for decades has enjoyed freedom and democracy and the right to choose its own destiny.

"We and the world cannot allow that freedom just to be snubbed out. We cannot and will not just look away."

Read more:
Johnson's statement to the nation in full

The PM said the mission of the UK and its allies was "clear", adding: "Diplomatically, politically, economically and - eventually - militarily, this hideous and barbaric venture of Vladimir Putin must end in failure."

He warned that Russian soldiers would "lose their lives" in the coming conflict and told the Russian people that Mr Putin's regime would be given "pariah status".

Russia intends to seize and control a large section of Ukraine, according to Western officials.

Their current assessment of Russian military movements indicates that Kyiv, Odessa, Mariupol and separatist areas in Ukraine's Donbas region could all be targeted.

They said it was unclear whether taking control of the whole of Ukraine was an immediate objective of the Kremlin.

PM brands Putin 'the Russian dictator'

In a message to Ukraine's 44 million people in his TV address, the PM said: "If the months ahead are grim and the flame of freedom burns low, I know that it will blaze bright again in Ukraine.

"Because for all his bombs and tanks and missiles, I don't believe that the Russian dictator will ever subdue the national feeling of the Ukrainians and their passionate belief that their country should be free."

Mr Johnson said the UK and its allies will work "for however long it takes to ensure that the sovereignty and independence of Ukraine is restored".

He also assured Britons that the government will "do everything to keep our country safe" after Mr Putin's threats "against those who stand with Ukraine".

Read more:
Putin launches invasion of Ukraine

The Russian president is suggested to have been making reference to nuclear war when, in comments as the attack on Ukraine began, he warned of "consequences you have never seen in history" to those who tried to impede him.

The PM described Russia's "act of wanton and reckless aggression" as an "attack not just on Ukraine" but "an attack on democracy and freedom in eastern Europe and around the world".

He added: "This crisis is about the right of a free sovereign, independent European people to choose their own future. That is a right that the UK will always defend."

Downing Street later clarified that Mr Johnson's reference to "militarily" facing down Russia was an "expectation that the Ukrainian people will fight, will engage with Russian forces".

The PM's official spokesman added that Britain was providing "defensive military capability" to Ukraine.

PM to unveil 'unprecedented sanctions' as Russian ambassador summoned

Mr Johnson was due to speak with fellow G7 leaders on Thursday and is calling for an urgent meeting of all NATO leaders as soon as possible following Russia's latest aggression against its neighbour.

He held calls with Norway's Prime Minister Jonas Store and German Chancellor Olaf Scholz.

Downing Street said Mr Johnson "described this morning's scenes as the most horrific act of aggression in his political lifetime" in his call with Mr Scholz.

At about 5pm, Mr Johnson will also address the Commons where he is expected to give details of more punitive measures on Russia.

Foreign Office minister James Cleverly earlier told Sky News that the UK would bring forward "an unprecedented level of sanctions" with the aim it will "hopefully persuade the people around Vladimir Putin this is completely the wrong thing to do".

Cabinet Office minister Stephen Barclay, also the PM's chief of staff, said Mr Johnson would unveil "overwhelming sanctions" when he speaks in the Commons.

These are likely to be coordinated with the US and EU, as well as other Western allies.

Meanwhile, Foreign Secretary Liz Truss summoned the Russian ambassador to meet with her on Thursday to "explain Russia's illegal, unprovoked invasion of Ukraine".

A Foreign Office source said: "It was a heated meeting. Liz kicked him out early.

"She said he should be ashamed of himself, that Russia has lied repeatedly and lost its last shred of credibility with the international community."

Home Secretary Priti Patel said the government would be "especially mindful of the potential for cyber attacks and disinformation emanating from Russia".

"Be in no doubt there is work ongoing across government 24/7 to maximise our resilience to any such attacks, which would be met with a suitably robust response," she posted on Twitter.

Mr Johnson chaired a meeting of the government's emergency COBRA committee at 7.30am on Thursday morning following a 4am call with Ukrainian President Volodymyr Zelenskyy.

Labour leader Sir Keir Starmer said Mr Putin's "bandit rule" must fail, adding: "This must be a turning point in our history.

"We must look back and say that this terrible day was actually when Putin doomed himself to defeat.

"He seeks division, so we must stay united. He hopes for inaction, so we must take a stand.

"He believes that we are too corrupted to do the right thing, so we must prove him wrong. I believe we can, but only if we stand together."

Tory MPs calls for NATO air support for Ukraine

The PM previously announced a "first tranche" of sanctions against Russia on Tuesday after Mr Putin initially ordered his troops to move into two separatist regions in eastern Ukraine.

Those sanctions targeted five Russian banks and three oligarchs, freezing their UK assets and banning travel to Britain.

However, the package was met with a lukewarm response in the Commons with many MPs - including senior Conservatives - calling on the PM to go further and faster in punishing Mr Putin's regime and those close to it.

After Russia's invasion of Ukraine in the early hours of Thursday, a string of Conservative MPs called for NATO countries to provide air support to the Ukrainian army.

"Whether we like it or not Putin has effectively declared war on the West," former Cabinet minister David Davis posted on Twitter.

"While we should continue to impose the most ferocious sanctions they are no longer a deterrent. They are a punishment. They will not stop him.

"If NATO does not act now, Ukraine will be defeated in a matter of days."

Mr Davis admitted it was "far too late to get boots on the ground", but added: "It is not too late to provide air support to the Ukrainian army, which may neutralize Putin's overwhelming armoured superiority."

Crispin Blunt, a former chair of the Commons foreign affairs committee, said the UK "must give Ukraine any help they need in the air and in cyber space".

Former minister Stephen Hammond called on the government to "work with our allies to impose tougher sanctions and a no-fly zone" over Ukraine.

Fellow Conservative MP Alicia Kearns also said the UK should "look to deploy air strength to protect Ukrainian air space to prevent mass casualties".