Read the full transcript of the prime minister's speech below:
"Shortly after 4 o’ clock this morning I spoke to president Zelensky of Ukraine to offer the continued support of the UK because our worst fears have now come true and all our warnings have proved tragically accurate.
"President Putin of Russia has unleashed war in our European continent. He has attacked a friendly country without any provocation and without any credible excuse.
"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.
WATCH: My address to the nation on the situation in Ukraine.https://twitter.com/i/broadcasts/1ZkJzbnVOPrJv\u00a0\u2026
"We – and the world – cannot allow that freedom just to be snuffed out. We cannot and will not just look away.
"It is because we have been so alarmed in recent months at the 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. And so I say to the people of Russia, whose president has just authorised a tidal wave of violence against a fellow Slavic people.
"The parents of Russian soldiers who will lose their lives. I cannot believe this is being done in your name or that you really want the pariah status it will bring to the Putin regime.
"And 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.
"And 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.
"And 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.
"We are joined in our outrage by friends and allies around the world. 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 is an attack on democracy and freedom in East Europe and around the world. This crisis is about the right of a free, sovereign independent European people to choose their own future and that is a right that the UK will always defend."
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