Ukraine have been deployed as part of Kyiv's ongoing incursion into Russia, British media reported on Thursday.
It is believed to be the first time the Challenger 2 tanks have been used in active combat on Russian territory, Sky News and other UK outlets said.
They did not specify how many of the tanks were thought to have been deployed by Ukraine in its surprise incursion into Russia's Kursk region, which began on August 6.
The UK Ministry of Defence declined to comment on what specific weaponry Ukraine was using, but noted «operations inside Russia» had not been precluded when the tanks and other arms were donated to Kyiv.
«There has been no change in UK government policy,» a ministry spokesman said.
«Under Article 51 of the UN Charter Ukraine has a clear right of self-defence against Russia's illegal attacks, that does not preclude operations inside Russia.
»We make clear during the gifting process that equipment is to be used in line with international law."
Ukraine sent troops and armoured vehicles into the Kursk region nine days ago, claiming the capture of several dozen villages in the most serious cross-border attack on Russian soil since World War II.
— 'Perfectly legitimate' -
Moscow on Thursday said it would send more troops to defend the neighbouring border region of Belgorod amid the ongoing Ukrainian assault.
British equipment supplied to Kyiv since Russia invaded more than two years ago includes Storm Shadow cruise missiles and a squadron of 14 Challenger 2 tanks deployed early last year.
T