Exclusive: Watch Moment RAF Jets Get Scrambled

When the alarm rings, they run.

The order, delivered over a secure line, is simple: "Scramble, scramble, scramble. Acknowledge."

In a building on the far side of RAF Coningsby, they turn and dash towards their aircraft. The fighter pilots have little or no idea what the threat is. There is no time for a briefing.

It might be a Russian bomber heading towards UK airspace, or a passenger aircraft approaching London.

It doesn't matter which. The adrenaline is pumping. They will get more information once they’re airborne, right now, seconds matter.

The training kicks in.

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This is one of the most sensitive ongoing operations in the British military and until now, cameras have never been allowed to every stage of the process. It is called QRA - Quick Reaction Alert. The RAF's mission to protect the skies of Britain.

The pilots can flick a switch to begin starting up the Typhoon jets, even as they climb into the cockpits.

The aircraft are armed and ready with short and medium range missiles.

As the threat is being assessed, the jets are cleared for take-off.

If it’s an emergency they will be given special permission to fly supersonic. From RAF Coningsby in Lincolnshire, they can be over London in 10 minutes. One of the busiest airspaces in the world.

"Climb flight level four one-zero,” is the instruction from the Air Command and Controlling centre at nearby RAF Scampton.

"Your mission, interrogate."

As they break through the clouds, the teams on the ground are gathering more information on the threat.

"He is still out of radio contact," is the latest update.

The team at the National Air Defence Operations Centre (NADOC) is based in a top secret Cold War nuclear bunker at Air Command in High Wycombe. They have a direct line to the Metropolitan Police Counter Terror unit and a dedicated section at the Department of Transport.

The three strands share intelligence on this aircraft to build a clearer picture of the threat.

"The role of the NADOC is very much to act as the hub, the fusion point for all the information coming in," Group Captain Martin Johnson tells us.

As they approach the suspect aircraft the Typhoons are careful not to spook it. They edge alongside the plane and sit just off both wings.

The pilots first try to contact the aircraft using hand or visual signals. They try to reach them on a radio channel.

They might also waggle their wings - an international sign to acknowledge recognition and compliance.

If that doesn’t work they can escalate to a show of force - flares, fired from the belly of the aircraft.

Finally, there is the ultimate threat. A chilling message, given calmly: "I am instructed by Her Majesty’s Government of the United Kingdom to warn you that if you do not respond immediately to my orders to turn east, you will be shot down."

In October 2014, a Typhoon pilot delivered this very warning to a Latvian cargo plane flying near London and not responding to air traffic control. IT was safely escorted to Stansted airport, one of the UK’s designated airports for terrorist incidents.

The decision to shoot down an aircraft would be taken by the Prime Minister or a senior Cabinet Minister.

It is something they rehearse on a regular basis. A red phone in the NADOC bunker at Air Command, connect straight to Downing Street. Pick it up and the voice on the other end answers with one word: "London."

"You know there are lives at stake. There are uncertainties," says Michael Fallon, the Defence Secretary.

"You’re unsure as to how real the threat is. There is advice available to you, but in the end, a Minister has to take that decision.”

If the order is given, the pilots wouldn't hesitate.

"We’re in the military. This is our job 24/7, 365 days a year," says Flight Lieutenant Jon, his surname withheld for security reasons.

"If that’s an order given to us and the correct protocols have been followed and the correct authentications have been given, then yes you can’t think about it. It’s our job."

:: Watch Britain's Top Guns - a Sky News exclusive report going inside the RAF - on Monday night at 9.30pm on Sky channel 501, Virgin Media channel 602, Freeview channel 132, and Freesat channel 202.