An EasyJet flight from Geneva was aborted moments before take-off after a passenger spotted a spanner still attached to the wing.
The packed passenger jet had already taxied to the runway when the 25-year-old Swiss man saw the tool wedged between the flaps which are raised or lowered to alter the lift of the aircraft.
He alerted cabin crew and the pilot immediately returned to the terminal so the adjustable monkey wrench could be removed.
The passenger, named only as Christophe, said: "I realised straight away that what I was seeing was not normal. There was a spanner attached to the wing."
An EasyJet spokesman said: "The captain returned to the plane's departure point and a spanner was discovered.
"We have opened an inquiry and the authorities have been informed."
After checks were made, the plane was able to set off for Copenhagen an hour later, EasyJet added.
An aviation expert told Switzerland's 20 Minutes online news website: 'With the vibrations from the acceleration, the tool could have fallen onto the runway, and then been hit by the next aircraft.
"It could have caused serious structural damage, just like with the Concorde crash in Paris.
"If it had stayed attached to the plane, the pilot would have realised when he retracted the flaps about 400 metres from the ground, and been forced to make an emergency landing."
The crash of an Air France Concorde in July 2000 which killed 113 people was believed to have been caused by a strip of metal from a Continental Airlines DC10 which fell onto the Charles de Gaulle airport runway and punctured the supersonic jet's tyres.
The blow-out sent rubber fragments flying into a fuel tank, setting it ablaze and sending the Concorde plunging into a hotel several miles away.
- Daily Mail