RAF Typhoon fighter jets were scrambled to intercept Russian 'Bear' bombers in for the second time in a week, it has emerged.
The Typhoons were sent from RAF Lossiemouth on Friday, only two days after a Russian bomber was escorted from UK airspace by two UK jets.
Air Vice-Marshal Gary Waterfall, who is in charge of UK air defence, said: "The Royal Air Force was formed to secure the skies over the UK, and it remains our main task.
"This week's news has shown yet again that the RAF's quick reaction alert is an essential element of our nation's security."
The incident comes after what Nato described as an "unusual" increase in the number of Russian military aircraft encroaching in EU airspace.
Also on Friday, Portugal scrambled F 16 jets to intercept Russian bombers in the international airspace along its coast.
Nato members have so far conducted 100 intercepts of Russian aircraft in 2014, nearly three times as many as last year, before tensions between Moscow and the West increased in the wake of the conflict in Ukraine.
Nato said: "The bomber and tanker aircraft from Russia did not file flight plans or maintain radio contact with civilian air traffic control authorities and they were not using on-board transponders.
"This poses a potential risk to civil aviation as civilian air traffic control cannot detect these aircraft or ensure there is no interference with civilian air traffic."