Keir Starmer said Vladimir Putin must be stopped as he refused to rule out British-supplied long range missiles being fired into Russia.
The PM called on world leaders at the G20 summit in Rio de Janeiro, Brazil, to "double down" and said Ukraine must be given "what is necessary" to stop Putin winning the war. He faces mounting pressure to allow President Volodymyr Zelensky to use Storm Shadow warheads, which have a 190 mile range, to be aimed at Russia.
It comes after US President Joe Biden gave Ukraine the green light to use some long-range US-made weapons - an act the Kremlin has branded "reckless" and "dangerous". Russia has vowed to retaliate if any missiles are launched into its territory, and accused the US of "pouring oil on the fire".
Asked whether allowing Ukraine to fire missiles into Russia would escalate the risk of World War Three, the PM said: "We have to support Ukraine in this, this is about sovereignty of Ukraine, but it's also about our own freedoms.
"That's why for 1,000 days we've stood with Ukraine and we'll continue to do so. Number one issue here at the G20 is to make sure we absolutely double down on support for Ukraine for as long as it takes."
But he said he is "not going to get into operational details" about what weapons Mr Zelensky will have at his disposal. "The only winner, if we were to do that, is (Vladimir) Putin," he said.
Delivering a firm message to Russia, which is also attending the G20, Mr Starmer said Putin cannot be allowed to win. Tuesday marks 1,000 days since the start of the blood-soaked despot's invasion.
The PM added: "I've been really clear for a long time now that we need to double down. We need to make sure Ukraine has what is necessary for as long as necessary, because we cannot allow Putin to win this war."
Asked if he had spoken to Sergey Lavrov, Russia's foreign minister, he said: "I haven't spoken to Russia and I've got no plans to do so." Mr Lavrov is attending the summit in Brazil as Mr Putin faces arrest charged with war crimes. The Russian President's spokesman, Dmitry Peskov, hit back at Mr Biden's surprise decision to allow the weapons' use.
He said: “This decision is reckless, dangerous, aimed at a qualitative change, a qualitative increase in the level of involvement of the United States.” Russia warned of an "appropriate response", saying: "Kyiv's use of long-range missiles to attack our territory would represent the direct involvement of the United States and its satellites in hostilities against Russia, as well as a radical change in the essence and nature of the conflict.
"Russia's response in such a case will be appropriate and tangible." A No10 spokeswoman earlier said the Government wanted to put Ukraine in the "strongest possible position going into the winter to defend its sovereign territory".
Mr Biden has reiterated the US government’s support for Ukraine - though he didn't refer directly to the decision to green light Ukraine's use of long-range missiles. We all have to work to end the conflicts and crises that are eroding progress to improve security around the world,” Mr Biden said.
He added: “The United States strongly supports Ukraine’s sovereignty and territorial integrity. Everyone around this table, in my view, should as well.”
Ex-Tory Defence Secretary Ben Wallace said allowing Kyiv to use the weapons would show the West "won't be bullied by Putin's threats of escalation".