Britain summoned the Russian ambassador on Thursday and asked him to explain why two Russian Bear long-range bombers had flown over the English Channel the previous day, forcing British authorities to reroute civil aircraft.

It was the latest in a series of known close encounters by Russian nuclear-capable planes buzzing Nato airspace with their transponders turned off, which governments have called an unacceptable risk to civil aviation.

A British government source told reporters the incident, which forced Britain to scramble Typhoon interceptor jets, was "a significant escalation" and marked a change in strategy since Russian aircraft had previously largely confined themselves to flying close to Scotland.