The Russian ambassador to the UK Andrey Kelin has been summoned after Moscow's violation of Nato airspace in Poland last week.
Poland shot down drones in its airspace last week with the backing of aircraft from its Nato allies.
It is the first time a member of the Western military alliance is known to have fired shots during Russia's war in Ukraine.
The incursions into Nato airspace were "utterly unacceptable", a spokesman for Britain's foreign office said in a statement.
Andrey Kelin has been summoned to the Foreign Office
|REUTERS
A Home Office spokesman added that the UK stands united with its Nato allies in condemning the "reckless actions".
They said: "Significant and unprecedented violation of Polish and Nato airspace by Russian drones last week, followed by a further incursion into Romanian airspace on Saturday, was utterly unacceptable.
“The UK stands united with Poland, Romania, Ukraine and our Nato allies in unreservedly condemning these reckless actions.
"As our continent once again faces the egregious expansion of Russia’s reckless behaviour, defence of Ukraine against Putin’s aggression is crucial to the security of the whole of Europe, including the UK."
LATEST DEVELOPMENTS FROM THE FRONTLINE IN UKRAINE
Poland shot down suspected Russian drones in its airspace last week
| REUTERS/DARIUSZ STEFANIUUkraine launched a large attack with at least 361 drones targeting Russia overnight, sparking a brief fire at the vast Kirishi oil refinery in Russia's northwest, Kremlin officials said late last night, reporting no injuries.
Alexander Drozdenko, the Governor of the Leningrad region, said three drones were destroyed in the Kirishi area and a fire sparked by falling debris had been put out.
Mr Drozdenko said no one was injured. Ukraine's drone command confirmed it attacked the refinery and said it had "carried out a successful strike."
In recent weeks, the United States has stepped up pressure on Nato countries to tighten energy sanctions on Russia in a bid to curtail its revenues and help end the war with Ukraine, a conflict President Donald Trump has struggled to bring to a close.
Secretary of State Marco Rubio said the incursion of Russian drones into Polish airspace twas unacceptable but that it remained unclear if Russia had deliberately sent the drones into Polish territory.
"We think it's an unacceptable and unfortunate and dangerous development," Mr Rubio told reporters before departing on a trip to Israel and Britain.
"No doubt about it: the drones were intentionally launched. The question is whether the drones were targeted to go into Poland specifically."
Mr Rubio said that if the drones were targeted at Poland, "if the evidence leads us there, then obviously that'll be a highly escalatory move."