Sir Keir Starmer has branded a Russian drone strike that hit homes in Romania a “serious violation of Nato airspace”.
Two people were injured in the attack on Thursday night in Galați, south-eastern Romania.
In a statement, the Prime Minister said: “Last night a Russian drone entered Romanian airspace and hit a residential building, injuring civilians.
“This is a serious violation of Nato airspace.
“Russia’s aggression against Ukraine and attacks on civilians and civilian infrastructure threatens the security of our entire continent. The UK unreservedly condemns such strikes.
“Time and again, Russia has shown it has no regard for civilian life, for international law, or for the sovereignty of its neighbours. That must not be allowed to stand.
“We stand shoulder to shoulder with Ukraine, with Romania, and with all our Nato allies in the face of continued Russian aggression.”
Romania borders Ukraine and is a member of the Nato military alliance.
General Brigadier Gheorghe Maxim said Romanian forces were unable to shoot down the Russian drone before it hit the apartment block because of its short flight time in the country's airspace and legal considerations.
The aircraft entered the country’s airspace at 1.52am local time and travelled 6.2 miles.
Romanian President Nicușor Dan on Friday said his administration would expel the Russian consul in the south-eastern city of Constanta and shut down the consulate.
The Russian consul is understood to have been declared "persona non grata".
Vladimir Putin is "aware" of the incident, the president's spokesperson Dmitry Peskov said according to Russian state media.
It comes after Ukraine also reported a series of Russian attacks overnight in which a school was destroyed and several people injured.
A Russian drone struck a school building in the Semenivska area of the Novgorod-Siversky district, according to Ukraine's state emergency service.
No injuries were reported.
But three people, including two teenagers, were hurt following explosions in the Kharkiv region.
In the Donetsk region, 16 residential buildings were reportedly damaged and one person was injured following a Russian airstrike.
Three districts of the Dnipropetrovsk region in Ukraine were also attacked and a 60-year-old man injured.
In the Volgograd region of Russia, a man was killed and two people were hurt in a Ukrainian drone attack, the area's governor Andrei Bocharov said in a post on Telegram.
