The British Ministry of Defence has said that two Russian warplanes "repeatedly and dangerously" intercepted an unarmed RAF spy plane over the Black Sea last month.
A Russian Su-35 fighter jet approached the reconnaissance plane, triggering its emergency systems and disabling its autopilot. A Su-27 jet also made six close passes by the British plane, passing just six metres from its nose.
Defence Secretary John Healy praised the "exceptional professionalism" of the RAF crew during the "unacceptable" Russian flights, which the Ministry of Defence said were the most dangerous Russian action since 2022, when a Russian pilot fired a missile at a British reconnaissance plane.
The Ministry of Defence said its aircraft had been on a routine international flight to help secure NATO's eastern flank.
Condemning the aerial interceptions, Healey said: "This incident is yet another example of dangerous and unacceptable behaviour by Russian pilots towards unarmed aircraft operating in international airspace".
The Ministry of Defence and the Foreign Office have called on the Russian embassy to condemn the incident.
The UK is trying to reassure Kiev that its new sanctions policy against Russia is not easing restrictions after Ukrainian officials warned the change could help Moscow fund the military efforts, a number of British media outlets reported.
This follows Britain's announcement that it would begin allowing imports of fuel made from Russian oil refined in third countries.
While Downing Street insists that the decision to allow temporary imports of Russian oil and jet fuel is just one element of a tougher sanctions package, a British minister has admitted that "things haven't been worked out".
On Wednesday evening, Ukrainian President Volodymyr Zelensky said that there had been discussions with the UK about the sanctions package and that Ukraine had "delivered its views".
Downing Street confirmed that Keir Starmer had spoken to Volodymyr Zelensky on the phone late last night. It also emerged that Foreign Secretary Yvette Cooper had spoken to her Ukrainian counterpart Andriy Sibiha, with British diplomats in Ukraine also trying to reassure officials in Kiev.
Says that long-term sanctions are "what most effectively affects Russia", Zelensky added that the issue was expected to be discussed further with the UK this week. The dispute began after the UK announced expanded sanctions against Russia, which came into force on Tuesday.
Initially announced in October, the sanctions banned Russian oil products processed in a third country. However, some short-term exemptions were added to the original plan as the war with Iran increased fuel prices. Jet fuel and diesel refined from Russian crude will be allowed temporarily, as will supplies of fossil gas from two Russian terminals. The licenses will be valid for three months before being reviewed.