The UK and Ukraine have not yet confirmed the use of the Storm Shadow missiles in Russia but their deployment has been widely reported in British media.
The UK had previously said that British tanks, anti-tank missiles and other military equipment could be used inside Russia as part of Ukraine’s defence – but had kept restrictions on the use of long-range missiles.
It comes just days after US President Joe Biden authorised the same policy shift.
Russia’s defence ministry said on Tuesday that Ukraine had fired six US-supplied Army Tactical Missile Systems (ATACMS) in the Bryansk region.
A Russian state news agency cited the ministry as saying the missiles caused no casualties or damage.
Mr Biden’s decision came after Russia began deploying North Korean ground troops to supplement its own forces in the conflict.
Russian politician Maria Butina and the son of Donald Trump, the US president-elect, both warned the move could spark the start of a third world war.
Vladimir Putin lowered the threshold required for the use of nuclear weapons after America’s decision, adding to fears that the conflict could escalate.
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