President Putin has ordered military operations in its neighbouring nation overnight.
Explosions have been reported across the Ukraine including in the west of the country.
The Prime Minister held a Cobra meeting this morning following the invasion.
Here’s what Mr Johnson said.
Will Boris Johnson address the nation?
The Prime Minister addressed the nation at midday.
It was shown on BBC, Sky and other news channels.
Earlier he has said the Russian military action in Ukraine was a ‘catastrophe for our continent’ and he called for an urgent meeting of Nato leaders.
What did PM announce and say in his address?
Prime Minister Boris Johnson said the UK and the world cannot allow Ukraine’s freedom to be ‘snuffed out’.
He said: ‘A vast invasion is under way by land, by sea and by air. And this is not, in the infamous phrase, some faraway country of which we know little.
‘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 snuffed out. We cannot and will not just look away.
“It’s because we’ve been so alarmed in recent months at Russian intimidation that the UK became one of the first countries in Europe to send defensive weaponry to help the Ukrainians.
‘Other allies have now done the same and we will do what more we can in the days ahead.
‘Today, in concert with our allies we will agree a massive package of economic sanctions designed in time to hobble the Russian economy. And to that end, we must also collectively cease the dependence on Russian oil and gas that for too long has given Putin his grip on Western politics.
‘Our mission is clear. Diplomatically, politically, economically, and eventually, militarily, this hideous and barbaric venture of Vladimir Putin must end in failure.’
He said the Putin regime would be condemned to ‘pariah status’ after unleashing a ‘tidal wave of violence’ against Ukraine.
In his address to the nation, the Prime Minister said the UK stood with Ukraine in its ‘moment of agony’.
‘I say to the Ukrainians in this moment of agony, we are with you. We are praying for you and your families, and we are on your side,” he said.
‘If the months ahead are grim and the flame of freedom burns low, I know that it will blaze bright again in Ukraine.
‘For all his bombs and tanks and missiles, I don’t believe the Russian dictator will ever subdue the national feeling of the Ukrainians and their passionate belief that their country should be free.’
Mr Johnson concluded by saying: ‘I say to the British people, and all who have heard the threats from Putin against those who stand with Ukraine, we will, of course, do everything to keep our country safe.’
He added: ‘We will work with them, for however long it takes, to ensure that the sovereignty and independence of Ukraine is restored.
‘Because this act of wanton and reckless aggression is an attack not just on Ukraine, it’s an attack on democracy and freedom in eastern Europe, and around the world.
‘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.’
Will the PM make a statement in the Commons?
He is also set to address MPs later today.
Mr Johnson will make a statement in the House of Commons at 5pm.
Will the UK introduce new sanctions?
It is expected that more stringent sanctions on Russia will be announced by the government.
Foreign Office minister James Cleverly said news that further sanctions are imminent has sparked a ‘huge reduction in Russia’s economic abilities to fund this invasion’.
He told BBC Radio 4’s Today programme: ‘The sanctions package that will be put in response to this is already actually having an effect. Just the announcement that it’s coming – we’ve seen the Russian stock market, the equivalent of the FTSE, drop by over 30%.
‘That is a huge reduction in Russia’s economic abilities to fund this invasion.
‘And those sanctions will be laid today and over forthcoming days to really prevent Russia from funding this invasion.’
A message from the Editor, Mark Waldron