British Challenger 2 tanks are thought to have been used in Ukraine’s surprise incursion into Russia.
It comes after the Government confirmed Kyiv was free to use UK weapons in its offensive in Kursk oblast.
The Ministry of Defence refused to comment for “operational” reasons but Sky News reported a UK source as saying that the battle tanks had been deployed by Ukrainian soldiers.
The Government said there had been no change in policy since the green light was given for UK weapons – with the exception of Storm Shadow missiles – to be used on Russian soil as part of Ukraine’s self-defence efforts.
The 82nd Air Assault Brigade, which has been operating British tanks since last year, has been involved in the ongoing incursion.
War-torn Ukraine has launched a surprise counter-offensive against Vladimir Putin’s Russia in recent weeks with the apparent aim of diverting Moscow’s forces away from land they have invaded.
The latest turn in the conflict has sparked concerns about a wider escalation of tensions between Russia and the West.
But the MoD has said Ukraine has a “clear right” to use weapons donated by the UK for its self-defence, and “that does not preclude operations inside Russia”.
The policy means anti-tank missiles, artillery, armoured vehicles and other weapons donated by the UK to Kyiv’s war effort could soon be seen on the battlefield in Russia.
However, the UK’s powerful Storm Shadow missiles will remain off limits for use beyond Ukraine’s borders.
Downing Street has previously insisted these long-range missiles are only to be used for conflict within Ukraine.
Ukrainian troops appear to be gaining more territory within the Kursk region in recent days while other neighbouring Russian states along Ukraine’s border have declared a state of emergency in response.
The offensive is believed to be the largest of its kind by a foreign army on Russian soil since the Second World War.