Western allies including Britain, France and the US, have taken a “big jump” towards a nuclear conflict by giving Ukraine permission to fire Western long-range missiles into Kremlin territory, a pro-Putin spokesperson, Sergey Markov, has said.

A regular Putin “mouthpiece”, Markov warned that the shock move by US President Joe Biden could mean that Britons could be facing a Christmas in shelters.

The threat, in an interview on the BBC Radio 4's The World At One, was also echoed by President-Elect Donald Trump’s son, Donald Trump Jnr, who suggested that those behind the move to approve Ukraine’s use of the missiles wanted to “make sure they got World War 3 going before my father has a chance to create peace and save lives”.

President Biden’s move, however, has been welcomed by Ukraine, by senior UK defence sources and by many European leaders.

The move will allow Ukraine to fire US ATACM missiles, UK Storm Shadow missiles and French Scalp missiles into Kremlin territory.

Those in favour of the move have noted that the Kremlin and its mouthpieces in the state-controlled media and academia had threatened nuclear war every time the West had stepped up its support for Ukraine, including when it provided tanks, fighter jets and other sophisticated weapon systems.

However, Markov, currently the Director General of Russia's Institute for Political Studies, was convinced the move was different as it would mean that Western militaries would be directly involved in the conflict for the first time - Ukraine would require their assistance to use the precision guided missile systems.

Speaking to BBC Radio 4, he said: “My reaction [to the White House’s decision] was awful, I couldn't sleep well because I am just afraid nuclear war is coming.

“This decision of United States, Great Britain and France is not a step towards nuclear war it is a big jump to nuclear war, nuclear catastrophe.

“It is a very dangerous decision. The point is these rocket-missiles cannot be fired by the Ukrainian army, technically.

“Because, these rocket-missiles need to have very good map of the Russian territory and only the US has this map, even Great Britain and France does not have this map.”

Markov added that the targeting of the rocket-missiles cannot be done by the Ukrainian army as it requires satellite-intelligence that the country does not have - instead they will be targeted to Russian cities by American, British and French militaries, he argued.

“So, we regard this as the start of a direct war by Britain, America and France against Russia, another nuclear power.

“American rocket missiles against our territory. This is a clear escalation of the war.”

Asked if Putin would use nuclear missiles in response, Mr Markov said 'not now' and said it was more likely that bases in eastern Europe could be hit by smaller, tactical nuclear missiles.

But he added: “I think Biden, Macron and Starmer want escalation which could lead to, from my understanding, in the worst scenario, the nuclear war happens before Christmas of this year.

“Probably you will not be able to say 'Merry Christmas' because you will stay in the hole trying to hide away [your] family from the nuclear catastrophe. It can develop very, very quickly.”