Sir Keir Starmer said Vladmir Putin started the war in Ukraine and claims the Russian leader could end the fighting straight away.
The Prime Minsters stern words come just hours before he holds talks with Joe Biden where Kyivs use of long-range Storm Shadow weapons is on the agenda.
Speaking to reporters on board his plane to Washington, Sir Keir said: There are really important developments likely in the next few weeks and months, both in Ukraine and the Middle East, and therefore a number of tactical decisions ought to be taken.
And he added: Russia started this conflict. Russia illegally invaded Ukraine. Russia can end this conflict straight away.
Ukraine has the right to self-defence, and obviously weve been absolutely fully supportive of Ukraines right to self-defence, providing training capabilities.
But we dont seek any conflict with Russia, thats not our intention in the slightest. But they started this conflict, and Ukraines got a right to self-defence.
Sir Keir Starmer speaks to reporters on his way to Washington DC to meet with President Joe Biden
Russian President Vladmir Putin speaks at Konstantin Palace on September 12. He warned the West it will be at war with Russia if it allows Ukraine to fire British missiles into his country
Western powers have, despite months of pleading from Kyiv, refused to let Ukrainian President Volodymyr Zelensky use their weapons to hit targets inside Russia, for fear of retaliation.
Currently Ukraines forces can use Storm Shadow missiles – which have a range of more than 150 miles – only inside their countrys borders.
But now it is thought Western leaders will, within weeks, announce they have relented, after concluding Russia was already escalating the conflict further by acquiring ballistic missiles from Iran.
If Ukraine was given the green light to aim Storm Shadows into Russian territory, it would allow its forces to hit the air bases from where attacks on its troops and civilians are being launched.
Asked about the move last night, the Prime Minister said he could not go into discussions about particular capabilities.
The PM and the US President are to have a two-hour summit in the White House today amid growing signs of a major shift in their stance on the war.
Vladimir Putin last night warned the West it will be at war with Russia if it allows Ukraine to fire British missiles into his country.
Foreign Secretary David Lammy (left) and Prime Minister Keir Starmer depart from Stansted Airport in Essex on September 12
The pair board the plane to fly to Washington DC and meet with the American President
President Joe Biden is set to meet with the British Prime Minister to discuss Kyivs use of long-range Storm Shadow weapons
Western powers have, despite months of pleading from Kyiv, refused to let Ukrainian President Volodymyr Zelensky (pictured) use their weapons to hit targets inside Russia
Ukrainian President Volodymyr Zelensky (centre) meets with US Secretary of State Antony Blinken (left) and British Foreign Secretary David Lammy (right) on September 12
Putin warned the widely expected decision to let Ukrainian forces fire the Anglo-French cruise missiles deep into his territory would change the very nature of the conflict.
It would mean Nato countries, the US, European countries, are at war with Russia, he told a state broadcaster yesterday. If thats the case, then taking into account the change of nature of the conflict, we will take the appropriate decisions based on the threats we will face.
US Secretary of State Antony Blinken further hinted at the new approach after visiting Ukraine, whose President Zelensky wants to present a victory plan to President Biden later this month.
Asked yesterday if the green light had been given for Kyiv to hit targets in Russia with Western weaponry, he replied: A hallmark of what weve done from day one – in fact, even before day one – of the Russian aggression against Ukraine in 2022 was to try to make sure Ukraine has what it needs when it needs it to deal with that aggression.
And as what Russia is doing has changed, as the battlefield has changed, weve adapted.
I can tell you . . .well adapt as necessary, including with regard to the means that are at Ukraines disposal to effectively defend against the Russian aggression.
Destroyed Russian tanks lie on the roadside near Sudzha, Kursk region in Russia on August 16, 2024
A public announcement on the change of approach by the West is not expected today, with more talks planned among European leaders in the coming weeks.
Foreign Secretary David Lammy, who will also be in the White House for todays talks, suggested on Wednesday the final decision will be taken at a summit in New York in a fortnight.
The UK-provided Storm Shadows, costing £2million apiece, could enable President Zelenskys forces to destroy airfields deep inside Russia and even hit a key naval base.