British Challenger Two 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.
In the last two weeks, Ukraine has carried out the biggest foreign attack on sovereign Russian territory since World War Two, which one Ukrainian official has said is designed to create a buffer zone to protect its population against attack.
Britain's then Foreign Minister Lord David Cameron said in May that Ukraine had a right to use weapons provided by London to strike targets inside Russia.
The MoD confirmed that Ukraine had a “clear right of self-defence against Russia’s illegal attacks"
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Setting out the use of weapons deliveries in some of the most explicit terms to date, a spokesman for Britain's Ministry of Defence said that Ukraine had a "clear right of self-defence against Russia's illegal attacks" and "that does not preclude operations inside Russia".
The spokesman added: "We make clear during the gifting process that equipment is to be used in line with international law."
The policy means that British tanks, anti-tank missiles, and other military equipment given to Ukraine can be used inside Russia as part of Ukraine's defence against Russia's invasion.
Restrictions on the use of British-supplied long-range Storm Shadow missiles, which can only be used within Ukraine's internationally accepted borders, remain in place.
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Former PM Rishi Sunak and Ukrainian President Zelensky meet Ukrainian troops being trained to command Challenger 2 tanks at a military facility in February
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The Ministry of Defence has declined to comment on the use of British tanks inside Russia.
Britain has pledged £7.6billion ($9.77billion) in military assistance for Ukraine since February 2022 when the Russian invasion began, according to the House of Commons Library, which publishes research.
This includes 14 of Britain's main battle tanks last year along with armoured vehicles, ammunition, air defence systems, electronic warfare equipment and artillery.
But Britain, like other Western governments, has so far refused to allow Ukraine to use long-range weapons because of the perceived risk of escalation in the conflict.
Ukrainian personnel hold a Ukrainian flag as they stand on a Challenger 2 tank during training at Bovington Camp, near Wool
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Earlier this week, President Zelensky said that Ukrainian troops were "moving further" into Russia while United States and Ukrainian officials said that Moscow had begun withdrawing some troops from Ukraine in an effort to repel the offensive.
The Ukrainian President said on social media: "In the Kursk region, we are moving further. From one to two kilometres in different areas since the beginning of the day."
Ukraine’s interior minister, Igor Klymenko, said: "The creation of a buffer zone in the Kursk region is a step to protect our border communities from daily hostile shelling."