Russian President Vladimir Putin threatened to strike the UK with a new ballistic missile after using the weapon to hit a target in Ukraine.
Downing Street condemned Mr Putin for further escalating the conflict by using a ballistic missile with a range of “several thousand kilometres” against the city of Dnipro.
Mr Putin suggested the missile could be used to hit Kyiv’s allies who have given Ukraine permission to use Western-supplied weapons to hit targets within Russia.
The UK is believed to have allowed its Storm Shadow missiles to be used by Ukrainian forces within the Kursk region of Russia, while the US has given permission for its ATACMS weapons to be fired at targets in Mr Putin’s country.
Mr Putin confirmed Russia has tested a new intermediate-range weapon, saying it came in response to Ukrainian strikes on the Russian territory with US and British missiles earlier this week.
The Russian leader declared that Russia would issue advance warnings before strikes on other countries to allow civilians to evacuate to safety.
“In response to the use of American and British long-range weapons on November 21 of this year, the Russian armed forces launched a combined strike on one of the facilities of the Ukrainian defence industry,” Mr Putin said in a televised address.
“One of the newest Russian medium-range missile systems was tested in combat conditions, in this case, with a ballistic missile in a non-nuclear hypersonic warhead.”
He added: “We consider ourselves entitled to use our weapons against military facilities of those countries that allow their weapons to be used against our facilities.”
In Westminster, the Prime Minister’s official spokesman said: “My understanding is that it is the first time that Russia has used a ballistic missile in Ukraine with a range of several thousand kilometres.”
No 10 said it was “an example of escalatory behaviour from Russia”.
But the Prime Minister’s spokesman added it “only serves to strengthen our resolve and to ensure that Ukraine has what it needs to act in self-defence against Russia’s reckless and illegal invasion”.
The missile’s range far outstrips that of newly authorised US and British supplied weapons.
The distance from Moscow to London is around 2,500km, suggesting the range of the new missile could threaten the UK.
Russia’s military claims it has shot down two British-made Storm Shadow missiles, following reports that debris from the weapons had been discovered in the country’s Kursk region, where Ukrainian forces have launched an incursion.
Defence Secretary John Healey earlier revealed to a committee of MPs that the UK knew Russia had been “preparing for months” to fire a new ballistic missile.
Mr Healey warned Ukraine faces a “serious moment” in its defence against Mr Putin’s invasion, but refused to confirm that Kyiv had been given permission to use Storm Shadow in Russia.
Downing Street and the Ministry of Defence have repeatedly declined to comment publicly on the use of Storm Shadow.
“It risks both operational security and in the end the only one that benefits from such a public debate is President Putin,” the Defence Secretary told MPs.
Battle lines in Ukraine are now “less stable than at any time since the early days of the full-scale Russian invasion”, Mr Healey said, citing British intelligence.
Speaking at the same time, Prime Minister Sir Keir Starmer told the House of Commons the UK “will not be deterred or distracted by reckless threats” from Mr Putin, who has lowered the threshold for using his nuclear arsenal.
Sir Keir also insisted that all the UK’s support for Kyiv was “in accordance with international law” and “always for self-defence”.
