Vladimir Putin threatens war with Nato as Starmer backs Ukraine's right to self-defence in Washington

The Prime Minister went on to say the UK does not “seek any conflict with Russia”, before adding: “That’s not our intention in the slightest.”

On Thursday, Mr Putin said allowing long-range strikes “would mean that NATO countries, the United States, and European countries are at war with Russia … if this is so, then, bearing in mind the change in the very essence of this conflict, we will make appropriate decisions based on the threats that will be created for us”.

Ukraine had issued a plea for restrictions to be lifted on using Western weaponry against Russian targets and the issue is likely to be discussed by Sir Keir in a meeting with US President Joe Biden at the White House tonight.

Foreign Secretary David Lammy and US secretary of state Antony Blinken were pressed on the subject during their visit to Ukraine earlier this week, but said they had to report back to their bosses, the Prime Minister and US President.

Responding to the Russian president’s remarks ahead of his meeting with Mr Biden yesterday, Sir Keir told reporters: “Russia started this conflict. Russia illegally invaded Ukraine. Russia could end this conflict straight away.

“Ukraine has the right to self defence and we’ve obviously been absolutely fully supportive of Ukraine’s right to self-defence – we’re providing training capability, as you know.

“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.”

Questioned on whether he believed it was a binary issue that the use of western missiles in Russian territory would constitute a Nato war with Russia, Sir Keir added: “First, to reiterate, it was Russia who started this in the first place. They caused the conflict, they’re the ones who are acting unlawfully.”

Concern about escalation has been one of the reasons why permission has not yet been given to Kyiv.

Yet reports from the US have suggested that President Biden has been softening his stance on this. It is thought this discussion will form a key part of the last minute meeting in the White House tonight.

It comes as the FSB security service said the Russian Foreign Ministry has terminated the accreditation of six employees in the British Embassy in Moscow’s political department. The move was in response to “numerous unfriendly steps” by London and after “signs of spying and sabotage” were detected, the FSB said in a statement.

A Foreign Office spokesperson said: “The accusations made today by the FSB against our staff are completely baseless.

“The Russian authorities revoked the diplomatic accreditation of six UK diplomats in Russia last month, following action taken by the UK Government in response to Russian state-directed activity across Europe and in the UK.

“We are unapologetic about protecting our national interests.” It is understood that the six diplomats left Russia weeks ago and are already being replaced.