Keir Starmer has said standing up to Russia is "about our way of life" as he called on Vladimir Putin to back down on his nuclear war threats.
The PM responded after the Russian tyrant approved a new nuclear doctrine as World War Three fears escalate. Asked how worried he is, the PM said Putin cannot be allowed to win - and vowed to give Ukraine whatever it needs.
The Kremlin said it would retaliate if long-range Western missiles were filed on Russian territory. Russian state media reports that Putin has approved a new nuclear doctrine approving the use of nuclear weapons if the country is "under threat".
Mr Starmer has refused to clarify whether he will authorise Volodymyr Zelensky to use British-supplied Storm Shadow missiles - with a range of almost 200 miles - against Russian territory. But in a hardening of his language he said Ukraine must be given "whatever is needed".
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."
In a separate interview with ITV he said: "It matters to Ukraine and sovereignty in Ukraine, but it also impacts every single person back in the UK. This is Russian aggression. We have supported Ukraine throughout this conflict.
"We will continue to do so because it's the right thing to do, and the single simplest way to end the conflict is for Russia to stop its aggression."
Asked if the UK will be providing Ukraine with long-range missiles, he said: "Well, my position has always been that Ukraine must have what it needs for as long as it needs. Putin must not win this war.
"But look, forgive me, I'm not going to go into operational matters, because there's only one winner if I do that, and that is Putin and it would undermine Ukrainian efforts."
Russia's new doctrine allows for a potential nuclear response even to a conventional attack on Russia by any nation supported by a nuclear power. This was branded the "latest example of irresponsibility" from "the depraved Russian government", by No10.