Prime Minister Sir Keir Starmer has told Ukrainian President Volodymyr Zelensky that British-supplied Storm Shadow missiles can be used to strike military targets inside Russia.
Speaking on his way to the Nato summit in Washington, the Labour leader said that Ukraine must use the long-range missiles in line with international law.
Sir Keir told reporters: "It is for Ukraine to decide how to deploy it for those defensive purposes."
He stressed the precision-guided weapons, which have a range of 155 miles, must be used "in accordance with international humanitarian law" and for "defensive purposes".
Defence Secretary John Healey confirmed the move, telling Sky News that “we will do all we can to help Ukraine in their fight to repel Putin's invasion".
He added: “We provide weapons equipment where we can for them to defend themselves, and as we do for ourselves and any other nation in conflict, we require, because it's international law, that war is conducted within those rules of the Geneva Convention.”
The announcement came during Sir Keir's meeting with Mr Zelensky at the Nato summit in Washington DC on Wednesday.
It marks a significant shift from the previous Conservative government's policy.
President Zelensky welcomed the decision, expressing gratitude for the UK's "unwavering support" in a social media post.
The Kremlin condemned the move. Vladimir Putin's spokesman Dimitry Peskov called it "an absolutely irresponsible step towards escalating tensions" and warned of a potential Russian response.
The announcement was made during the Nato summit marking the alliance's 75th anniversary.
In a joint declaration, Nato members said that Ukraine is on an "irreversible" path to joining the alliance.
The two-day summit, attended by leaders of the 32 member states, concludes on Thursday.
Opening the summit, US president Joe Biden declared the alliance “more powerful than ever” as it faced a “pivotal moment” and pledged to provide Ukraine with five new strategic air defence systems to counter Russian attacks.
Mr Biden said: “The war will end with Ukraine remaining a free and independent country. Russia will not prevail.”
