Britain says two Russian long-range bombers have flown over the English Channel and disrupted civilian air flights.
The British Foreign Office said it summoned the Russian ambassador to "account for" the January 28 incident, in which the Tupolev Tu-95 jets flew close to British airspace.
It said British authorities were forced to divert civilian aircraft because of the bombers.
British Typhoon fighter jets were scrambled to intercept the Russian aircraft and escort them "throughout the time they were in the U.K. area of interest."
The Foreign Office said the incident was "part of an increasing pattern of out-of-area operations" by Russia.
NATO has also reported increased incursions by Russian military aircraft following months of tension over Ukraine.
Former Soviet leader Mikhail Gorbachev said on January 29 that he fears the confrontation between Russia and the West over Ukraine could escalate into war.