'You will be shot down': What RAF typhoon pilot told Latvian cargo plane after it sparked terror alert by veering off course over London forcing fighter jets to scramble

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Two RAF fighters which scrambled across Britain to intercept a foreign aircraft apparently warned the plane would be 'shot down', after it sparked a terror alert by taking an unscheduled detour over London.

The Typhoons from RAF Coningsby, Lincolnshire, flew to Kent, south east England, at supersonic speeds to dramatically halt the plane on its way to Birmingham airport after it 'caused concern' to air traffic controllers. 

The swoop, which resulted in the Lativan-registered plane being grounded at Stansted airport, sparked a massive sonic boom which was heard across a 50-mile radius. 

Soon after the incident, chilling audio emerged appearing to show one of the RAF pilots warning the three-man crew on board the foreign plane to listen to military instructions or risk being 'shot down'.

Scroll down for audio and video

A Russian made Antonov An-26 cargo airliner (pictured) was escorted into Stansted Airport while flying over London after 'causing concern' to British air traffic controllers this afternoon 

An aviation expert said controllers 'probably knew' what the aircraft was - but that nobody would be prepared to take any chances incase it was 'another 9/11 on the Houses of Parliament'.

In the audio footage - which has not been verified - a man is heard saying: 'I’m instructed by Her Majesty’s government of the United Kingdom to warn you if you do not respond you will be shot down’.

The sound clip - which claims to be the voice of a pilot from one of the RAF pilots - was published on social media by Twitter user @RAFConingsbyInfo, an account which is not officially connected to the airbase.

The Ministry of Defence refused to comment on whether or not the clip was legitimate.  

The drama unfolded after the flight, which took off from Italy this morning, took an unprecedented detour when it approached London.

Flight mapping experts said it flew towards the centre of the capital, prompting the fighter jets to be launched from the airbase 160 miles away.

The planes were part of the Quick Reaction Alert system, which is designed to respond to 'aviation security incidents'. 

Aviation expert David Learmount, the Operations & Safety Editor at Flightglobal, said such incidents only happen in a 'minute' proportion of flights but trigger a dramatic response because of the possibility of a terror-related incident.

He said it was now confirmed that the aircraft had dialled into the wrong frequency when trying to contact UK airspace. But nobody would have wanted to take any chances at the time, he said.

He told MailOnline:  'They probably know who is in the airspace, but nobody is going to take any chances.

'In these instances, it's most likely that the wrong frequency has been dialled into - which happened in this case - but there is another possibility; the possibility that there is a hijacker hiding down the back of the plane, who has decided: "I am going to do another 9/11 on the Houses on Parliament".

'The RAF was reacting against this threat, which is normal when an ordinary flight with a flight plan suddenly goes incommunicado and they don't know why. 

The flight, pictured grounded at the airport, was on its way to Birmingham airport when it was intercepted 

'They have to assume the worst - especially if its did end up heading towards Buckingham Palace or Houses of Parliament.'

He added: 'The RAF is certainly not going to wittingly shoot down the flight with passengers unless convinced they would die in a terrorist attack.' 

A Stansted airport spokesman confirmed the aircraft had lost communications with air traffic controllers, leading to the interception over Sevenoaks, Kent. 

The aircraft - a Russian made Antonov An-26 cargo airliner - was deemed safe by officials after it was checked. It is due to continue its flight to Birmingham later this evening. 

The Latvian company RAF-Avia, which operated the plane, said the plane had been on a routine flight when the pilots lost communication with air traffic control when it arrived in UK airspace.

The plane, which was carrying automotive equipment, had taken off at 11am from Toscara in Italy, before landing in Vatry in France and then continuing its journey to the UK.  

This shows the flight path of the plane, which appeared to take a dramatic detour as it crossed London

Two Typhoon jets were deployed from RAF Coningsby, Lincolnshire (file pic) after fears were sparked 

The spokesman said it was 'probably some kind of malfunction or some emergency situation'.

This afternoon's incident triggered a huge explosion-like sound, which stretched across Kent, West Sussex and as far away as Hertfordshire and Essex. 

After the incident, the RAF tweeted: 'Apologies if we startled you during the successful intercept this afternoon - we only go supersonic over land when absolutely necessary.'

Twitter users from as far away as Dover on the south coast and Otford, in neighbouring West Sussex, tweeted about hearing the huge noise and windows shaking. 

Paul Dunlop tweeted: 'Jesus - Massive explosion-like noise heard down here in Sevenoaks. Whole house shook. Can see people saying they heard it across Kent.' 

Cleyton Tierney wrote: 'Huge bang shook the house, no idea what that was. Anyone else hear/feel it?' 

Ben Basten added: 'Who else felt the earth move and huge explosion in Kent 5 minutes ago? Is the world coming to an end? My house rocked.'  

Two RAF fighters triggered a massive sonic boom across Kent this afternoon when they intercepted a foreign aircraft. Pictured: Stansted

An RAF spokesman said: 'Typhoon aircraft were launched this afternoon from RAF Coningsby to intercept a civilian aircraft south of London which was causing concern to air traffic control authorities.

'The aircraft was safely escorted to London Stansted. To fulfill their quick reaction role the Typhoons were cleared to travel at supersonic speed, any noise disturbance as a result of this is regretted. Essex Police now have the lead.' 

The spokesman added that the jets were on their way back to RAF Coningsby around ten minutes later. 

Stansted Airport confirmed that the jets accompanied the plane to their airport, but have not revealed why. 

Kent Police confirmed the noise was created by RAF activity. 

A spokesman for Essex Police said: 'Military jets were deployed on a precautionary basis when a Latvian registered cargo plane was diverted to Stansted.

'This was due to a loss of communication with the aircraft. The aircraft landed safely at Stansted airport at around 5.20pm.

'All three people who were on board have been spoken to by police, it was established that everything was in order and reason for the short loss of communication was due to a change in airspace jurisdiction.

'Essex Police is not investigating the incident and the aircraft will be able to continue its journey to Birmingham when it is ready.' 

According to the company's website, RAF-AVIA, which was set up in 1990, transports 'all kinds of dangerous goods', including radioactive materials.   

A sonic boom is the sound associated with the shockwaves created when an object, such as an aircraft, breaks the sound barrier.

In April, 2012, a massive sonic boom was heard across five counties from Somerset to Oxfordshire after two fighter jets were scrambled to help a helicopter, after its pilot accidentally emitted an emergency signal.