The development comes with warnings that the UK and Russia are now "one step closer" to being at war.

RAF Tornado pilots have been instructed to avoid contact with Russian aircraft while engaged in missions for Operation Shader — the codename for the RAF's anti-ISIS work in Iraq and Syria.

But their aircraft have been armed with air-to-air missiles and the pilots have been given the green light to defend themselves if they are threatened by Russian pilots.

"The first thing a British pilot will do is to try to avoid a situation where an air-to-air attack is likely to occur — you avoid an area if there is Russian activity," an unidentified source from the UK's Permanent Joint Headquarters told the Sunday Times. "But if a pilot is fired on or believes he is about to be fired on, he can defend himself. We now have a situation where a single pilot, irrespective of nationality, can have a strategic impact on future events."