An EasyJet flight was moments from taking off when a passenger glanced out of his window and noticed a spanner in the wing.
The plane, which had already taxied down the runway, was about to depart on a flight from Geneva to Copenhagen.
The passenger, known only as Christophe, alerted the pilot and the flight was halted.
The 25-year-old from Switzerland said: “I realised straight away that what I was seeing was not normal.
“There was a spanner attached to the wing.”
An hour later the flight set off for Denmark after being given the all clear.
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.”
An aviation expert told Switzerland’s 20 Minutes online news website that if the spanner had fallen on the runway, it could have been hit by the next aircraft.
“It could have caused serious structural damage, just like with the Concorde crash in Paris,” the said.
“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.’