A flight from Heathrow to San Francisco was delayed for several hours after a mouse was spotted on the plane.
Passengers on the 10:40 GMT British Airways flight were strapped in and waiting to take off when the delay was announced.
Carly, who was on the flight, said the air crew announced “a rather unusual occurrence has occurred”.
They said rules stipulate planes cannot take off with mice onboard and another flight would be laid on.
“I’m not sure people knew how to react,” said Carly.
“There was general disbelief.”
British Airways said: “We know almost everyone wants to fly with us to San Francisco, but on this occasion there was one very small customer who we had to send back to the gate.
“Everyone with two legs is now on their way to California, and we are sorry for the delay.”
Heathrow Airport said it could not comment on individual flights.