AN investigation has been launched into the helicopter that sparked a hijack scare and led to Thursday night’s sonic boom.

Two Typhoon fighter jets were scrambled over southern England at about 6pm on Thursday because of an emergency alarm being broadcast from a civilian helicopter.

One of the jets was ordered to travel at supersonic speed, creating a series of sonic booms heard from Birmingham, through much of Oxfordshire and down to Southampton.

Yesterday the RAF released more details about the scare and said an investigation had been launched.

The two Typhoons were part of the air force’s Quick Reaction Alert team based at RAF Coningsbury in Lincolnshire, an RAF spokesman said. They were scrambled because the helicopter was transmitting on an emergency frequency and not in radio communication with Air Traffic Control.

The spokesman added: “Authorisation was given for one of the Typhoons to transit at supersonic speed over land, which resulted in the sonic boom heard by the public.”

The Typhoons intercepted a civilian Gazelle helicopter in the vicinity of Bristol, and, after identification, the helicopter successfully re-established communications with Air Traffic Control.

“The reason for the emergency signal and communications failure is yet to be determined.

“The MoD will be providing all available information on the intercept to the Department for Transport, which will lead on the investigation process in order to complete any follow-up action if considered appropriate, and to also try and reduce the chance of any repetition of this type of incident.”

Police in Oxfordshire were inundated by calls from people who feared there had been an explosion.