GETTY The Russian crew had started the countdown to arm a nuclear bomb, when intercepted near the UK

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The discovery was made after RAF specialists analyzed a four-second signal transmitted from one of the Tupolev Tu-160 bombers, known by Nato as "Blackjacks", in the days following Thursday's incursion. Analysts at RAF Boulmer, Britain's Control and Reporting Centre, confirmed that the Russian bombers had begun the sequence to arm nuclear weapons while carrying out the incursion. It is not the first time they have done this and comparison with a similar signal transmitted by a TU-95 "Bear" bomber revealed Russian air crew had begun the countdown during an incursion last year, as well. The Sunday Express revealed that the bomber involved in the February 2014 incident had been carrying a submarine-busting nuclear depth charge designed to attack Britain's Trident-carrying Vanguard submarines. "All I can say is that we now know it related to the first stage of arming a nuclear device," said a senior RAF source last night.

"There are several additional arming procedures which, thankfully, were not carried out." Last week's air incursion, which was intercepted by Two RAF Typhoon jets, was the seventh this year by Russian forces. RAF Boulmer, in Longhoughton. Northmberland, is the headquarters of the Air Surveillance and Control System force and the nerve centre of UK air security operations. Once a target is tracked the information is fed into the NATO network operated by the Combined Air Operations Centre at Uedem, Germany and the RAF's Air Defence Operations Centre at Air Command, High Wycombe. However it was only when a female analyst decoded last week's transmission that it was realised Russia had carried out a similar exercise last year. Aviation expert Justin Bronk, of the Royal United Services Institute think tank, said the revelation confirmed a "worrying" development in Russian strategy. "Putin does not allow his air force to fly with cruise missiles because he is so worried about defections, but dual purpose bombs such as nuclear depth charges are a different matter. And these are designed to be a direct threat to or nuclear deterrent.

"It is entirely probable that Russian crews have been practicing arming drills. The whole process can take several minutes, and it is important to be able to carry out quickly." He added that the lack of Nimrod maritime surveillance patrol aircraft made the situation more worrying. Britain's Nimrod fleet, which was scrapped as part of the SDR in 2010, used to perform so-called "delousing" exercises, whereby it would give advanced warning of Russian aircraft when Trident-carrying Vanguard submarines set off on patrol.

Putin does not allow his air force to fly with cruise missiles because he is so worried about defections, but dual purpose bombs such as nuclear depth charges are a different matter Aviation expert Justin Bronk