Airport authorities said in a statement that the 151 passengers on board disembarked the plane using emergency slides and were taken to a transit gate where they were checked by police.

There are reports that police also destroyed a piece of baggage that could not be assigned to anybody on board.

Take-offs and landings were suspended for three hours, causing 10 flights to be diverted to other airports and delays to over a dozen more.

A spokesman for Cologne-Bonn airport said: "A plane on the way from Ljubljana to London has landed this evening around 6.30pm locla time in Cologne unplanned.

"Prior to this, the pilot had been informed about a suspicious conversation on board, after which he decided to make an unscheduled landing in Cologne/Bonn.

"After the safe landing of the aircraft of the type A319, the 151 passengers left the machine via emergency slides and were taken to a transit gate.