Two Ryanair airliners were involved in a near miss while over northern Spain, according to investigators.
The incident took place earlier this month close to the city of Pamplona.
Details of the near miss are contained in a preliminary report into the incident by the spanish Civil Aviation Accidents and Incidents Investigation Commission.
According to the report the incident – which took place on October 2 – a Boeing 737-800 plane travelling from Santiago to Palma de Mallorca, and an identical aircraft heading from Sevellie to Toulouse, came dangerous close to one another.
The planes came so close to each other that the anti-collision alert systems on both aircraft sounded.
Investigators says the two planes were around four kilometers apart horizontally, and just 122 metres apart vertically.
After the alarms sounded the flight crew on the aircraft changed course and the flights continued without further incident.