Russian state TV has claimed military targets on UK soil should be attacked if British soldiers are deployed in Ukraine.
Igor Korotchenko, a pro-Kremlin military journalist, said Russia doesn't "desire" war with the UK, but would react harshly if any soldiers were sent to the frontline.
Speaking on the state-owned Russia 1, Korotchenko said Russia "reserved the right to resort to any possible action" if British soldiers engaged with the Russian armed forces in Ukraine.
Britain has been an ally of Ukraine since Vladimir Putin ordered an invasion on 24 February, providing more than £2bn in weapons and aid to the country.
There has been no suggestion of UK forces putting boots on the ground. In March, prime minister Boris Johnson said he wanted to make it "crystal clear" that "we will not fight Russian forces in Ukraine" and that he would keep troops within the boundaries of Nato countries to prevent the crisis escalating even further.
State TV is a powerful propaganda tool used by the Kremlin, spreading disinformation and extreme opinions to shape public opinion.
Korotchenko said that if any Nato country's servicemen were to offer "resistance" to what they are referring to as "the special military operation", it would give "Russia the right to respond in any way, including preventative strikes."
He added: "There's no need for immediate nuclear strikes. We have three systems carrying Kinzal missiles.
"So when Britain finally decides to make war with Russia, let us deliver three demonstrative strikes on London, not on civilian targets, but on the UK Ministry of Defence, where they make plans and are designing a war against Russia.
"Against British military bases where, among other things, US nuclear weapons are stored.
"And on other decision-making centres in London.
"This is not a threat, this is a preventative right to self-defence which we must exercise without fail should Britain really decide to fight us."
On Thursday Russia said it will only use nuclear weapons in "emergency circumstances" after facing several setbacks in Ukraine that has seen its offensive stall.
The Russian foreign ministry said on Thursday that Moscow has no interest in a direct confrontation with Nato and the United States and it had no need to use nuclear weapons during its military campaign in Ukraine.
The ministry dismissed media speculation that Moscow might deploy nuclear or chemical weapons in the conflict as "absolute lies."
Volodymyr Zelenskyy has claimed Putin could order something "particularly ugly" this week, as Ukraine prepares to celebrate its 31st independence day on Wednesday.
The annual celebration on 24 August also marks exactly six months since Putin ordered the invasion on Ukraine.
"All of Ukraine's partners have been informed about what the terrorist state can prepare for this week," Zelenskyy said in a nightly video address, referring to Russia.
"We must be aware that this week Russia may do something particularly disgusting, particularly cruel, such is the enemy of ours.
"But in any other week during these six months Russia has done the same thing constantly, disgusting and cruel."