A Ryanair flight travelling from London Stansted to Sofia was diverted to Plovdiv, Bulgaria, following a reported bomb threat on board.
Audio from the cockpit reveals air traffic controllers warning the pilots that a military aircraft would follow them into Bulgarian airspace.
The pilots were also told that two other aircraft had already landed in Sofia following similar threats, leaving the airport unable to handle additional diverted flights.
The incident comes as around 20 flights across Europe were affected by hoax bomb alerts for a third consecutive day.