On the BBC's Today Programme on Radio 4, Sergei Markov claimed "your cities will be targeted".
The professor of international relations at Moscow University, ex-Kremlin man and former adviser to Putin himself said: "It's not a good morning for everybody.
"In Russia there's partial mobilisation and for your British listeners, Vladimir Putin told you that he would be ready to use nuclear weapons against Western countries... including against Great Britain."
The UK, along with its NATO allies, has been supplying Ukraine with weapons systems in defence against the barbaric Russian full-scale invasion in February.
"If Great Britain will continue to be aggressor against Russia... if Prime Minister of Great Britain Liz Truss still has a plan to destroy Russia, people in London should understand the threat [of nuclear weapons]," Markov added, accusing the West of using Ukrainian soldiers as 'slaves' to weaken the Russian army.
"It was absolutely clear that Russia has no war against Ukraine...there's no reason to use tactical nuclear weapons against Ukrainians.
"But Ukraine is occupied by Western countries... it's Western countries fighting against Russian army, using Ukrainian soldiers as their slaves. This was the main idea of Vladimir Putin."
Economic sanctions against Russian businesses and oligarchs are "profoundly weakening Russia’s economy", according to the US embassy in Georgia, which has also borne the brunt of Russian aggression in recent years.
Now, Russian troops are on the back foot and have failed to meet most, if not all of Putin's key objectives in the war in Ukraine.
This has raised concerns about the possibility of further escalation in the war. Today, Putin announced the mobilisation of 300,000 reservists to be called up to fight and claimed he was "not bluffing" about the possibility of using nuclear weapons.
Russia is also planning to hold referendums, which will be little more than an undemocratic sham, in the regions of Donetsk, Luhansk, Zaporizhzhia and Kherson. If annexed, any more attacks on the territory could be deemed as aggression on the Russian homeland.
He said: "If the territorial integrity of our country is threatened, to defend Russia and our people, we will use all means we have. This is not a bluff.
"The territorial integrity of our motherland, our independence and freedom will be secured, I repeat, with all the means we have. Those who try to blackmail us with nuclear weapons should know that the prevailing winds can turn in their direction."
Despite Putin being the one the talk of actually using nuclear weapons, Markov said any nuclear war would be the "result of crazy behaviour of US president Joe Biden and prime ministers of Great Britain Boris Johnson and [Liz] Truss".
In mainland Russia, a whipping up of censored propaganda is vastly in favour of the 'special operation' in which Russia claims it is defending its homeland from the 'threat' of a Nazi state.
These are false claims.
Those in Russia who protest against the war can be slapped with up to 15 years in prison if they speak out against Putin's