Storm Shadow missiles supplied to Ukraine by Britain have been used in Russia, according to reports.
Wreckage from one of the long-range weapons has been found in Russia’s Kursk region, which borders Ukraine, several media outlets have said.
Storm Shadow’s use in Russia comes a day after Moscow said American long-range weapons had been fired into its territory.
Russian President Vladimir Putin has lowered the threshold for using nuclear weapons, after US President Joe Biden gave Kyiv permission to fire US ATACMS long-range missiles into Russia.
Downing Street would not comment on the reports.
No 10 has previously refused to comment on any changes in policy about allowing Kyiv to use long range missiles in Russia.
Earlier on Wednesday, the US closed its embassy in Kyiv because of a “potential significant air attack” by Mr Putin’s forces.
