Keir Starmer has lashed out at "irresponsible rhetoric" from Russia when asked if Brits should prepare for nuclear war.
The PM said he will not back down in the face of Vladimir Putin's threats. Speaking at a press conference at the G20 summit in Rio de Janeiro, Brazil, Mr Starmer said: "There's irresponsible rhetoric coming from Russia and that is not going to deter our support for Ukraine."
He called on the warmonger Russian President to back down as tensions rise over Ukraine's use of long-range weapons. Mr Starmer faces calls to give Volodymyr Zelensky the green light to use British-supplied Storm Shadow missiles, which have a range of nearly 200 miles.
Putin has not been present at the summit as he could face arrest after an International Criminal Court warrant was issued. Mr Starmer described him as "the author of his own exile".
He said: "For the third year running Putin did not attend this summit. He is the author of his own on exile, and on day 1,000 of Russia's illegal war in Ukraine I say it again, get out of Ukraine."
He said he has been arguing with world leaders calling for them to "double down" on support for Ukraine. He said: "We have stood with Ukraine from the start and I've been doubling down in my clear message that we need to ensure that Ukraine has what is needed for as long as it's needed to win this war against Putin."
It comes after Vladimir Putin promised to retaliate if Western long-range missiles were fired into Russian territory. The Kremlin is reported to have adopted a new nuclear doctrine giving it greater powers to use the weapons in response to attacks.
The summit, which brings together the leaders of the world's biggest economies, has been dominated by discussions about UK-supplied missiles. Mr Starmer has repeatedly refused to say whether he will grant President Volodymyr Zelensky's pleas to be able to fire Storm Shadow warheads into Russian-controlled areas.
Earlier the PM said standing up to Russia is "about our way of life" as he called on Putin to back down on his nuclear war threats. Asked how comfortable he is that Brits could be put at risk, Mr Starmer told Sky News: "Well, it's very important that we're steadfast in our support for Ukraine. Russia is the aggressor and Russia has to be the one that makes the move to stop this war, but we must support Ukraine. It's impacting not just Ukraine. It's impacting the rest of the world, including the UK."
And speaking on the 1,000th day since Putin's invasion he said: "This is a thousand days of conflict and there is a very high cost if Russian aggression is seen to pay off. A cost that we will all pay, because this is not just about sovereignty in Ukraine, it's about our freedoms.
"It's about our way of life and it impacts everybody across the United Kingdom. But, you know, we are working of course with allies to make sure Ukraine is in the best possible position."