Sir Keir Starmer said Vladimir Putin started the war in Ukraine and claims the Russian leader could end the fighting 'straight away'.

The Prime Minster's stern words come just hours before he holds talks with Joe Biden where Kyiv's 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 we've been absolutely fully supportive of Ukraine's right to self-defence, providing training capabilities.

'But we don't seek any conflict with Russia, that's not our ­intention in the slightest. But they started this conflict, and Ukraine's got a right to self-defence.'

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 Ukraine's forces can use Storm Shadow missiles – which have a range of more than 150 miles – only inside their country's 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.

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 that's 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 we've 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, we've adapted.

'I can tell you . . .we'll adapt as necessary, including with regard to the means that are at Ukraine's disposal to effectively defend against the Russian aggression.'

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 today's 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 Zelensky's forces to destroy ­airfields deep inside Russia and even hit a key naval base.