LONDON: 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, a move that forced British authorities to divert civil aircraft.

"The Russian planes caused disruption to civil aviation. That is why we summoned the Russian ambassador today to account for the incident," the foreign office said in a statement.

Typhoon fighter jets were scrambled to intercept the two Russian planes, which were flying close to UK airspace, on Wednesday.

The foreign office said the incident was part of an increasing pattern of "out of area operations" by Russian aircraft, noting the planes had stopped short of entering UK airspace.

A British government source told Reuters the Russian move was viewed as "a significant escalation" and was a change in strategy since Russian aircraft had previously largely confined themselves to flying close to Scotland.

"It was very dangerous. Civil aircraft flying to the UK had to be rerouted," the source said. "The Russians were flying with their transponders turned off so could only be seen on military radar. They haven't flown this far south before."