Long-range British Storm Shadow missiles have reportedly been fired into Russia by Ukraine for the first time.
The weapons were fired into the Kursk region on Wednesday morning, it is understood.
The strikes were approved in response to Moscow’s escalation by deploying North Korean troops in its war against Ukraine, an unnamed Western official told Bloomberg.
Defence Secretary John Healey did not deny that the British missiles had been fired on Russia when questioned about it in the Commons on Wednesday afternoon.
He said: “We, as a nation and as a Government, are doubling down on our support for Ukraine and determined to do more.
“I discussed this with [Ukrainian Defence Minister] Rustem Umerov in a call yesterday where he talked about the robust response that Ukraine is making to recent Russian escalations.
“That's the escalation in the attacks on Ukrainian cities and children, the escalation in attacks on their energy system, and the escalation with deploying 10,000 North Korean troops in combat positions on the front line.”
It comes after US president Joe Biden agreed to allow Kyiv to use American long-range Atacms weapons in Russia.
A spokesman for Prime Minister Sir Keir Starmer said the Government would not be commenting on the reports or operational matters.
However, Sir Keir said on Tuesday said that the UK recognised it needed to “double down” on its support for Ukraine.
Speaking to media at the G20 summit in Brazil, Sir Keir said there was a need to do more to help the country after its electricity network was damaged by a wave of Russian bombing over the weekend.
“I’ve been really clear for a long time now, we need to double down,” Sir Keir said.
“We need to make sure Ukraine has what is necessary for as long as necessary, because we cannot allow Putin to win this war.”
But he refused to say what permission he may or may not have given to Ukrainian President Volodymyr Zelensky.
He added: "Obviously, I'm not going to get into operational details because the only winner, if we were to do that, is [Russian President Vladimir] Putin and I'm not prepared to do that," he said.
Kyiv used US-supplied Atacms to hit a military facility in Bryansk early on Tuesday.
Russian President Vladimir Putin responded by lowering the threshold for firing nuclear weapons at Ukraine and its allies.
It advised American citizens to be prepared to move immediately to a shelter if there was an air raid warning. It later indicated the embassy would be open on Thursday.
The British embassy in Ukraine remains open.
The Biden administration will also allow Ukraine to use American-supplied anti-personnel land mines to help fight off Russian forces, US Defence Secretary Lloyd Austin has said.
Speaking to reporters during a trip to Laos, Mr Austin said the shift in policy follows changing tactics by the Russians.
He said Russian ground troops are leading the movement on the battlefield, rather than forces more protected in armoured carriers, so Ukraine has "a need for things that can help slow down that effort on the part of the Russians".
