British Airways was forced to apologise after a pilot accidentally activated the emergency slide.
Flight BA886 was just about to leave for Romania from London Heathrow when the pilot opened the door without disarming it, causing the slide to deploy.
The mistake delayed the flight for hours and cost the airline around £50,000.
Passengers were in their seats and waiting to take off when the pilot, who was believed to be experienced, made the mistake.
BA was forced to apologise to flyers
PA
A source told The Sun: "The captain of all people would have known the safety rules around opening doors. This is beyond a rookie error. It’s totally baffling to understand."
The flight, scheduled to depart at 9.55am on Saturday, did not leave until 1.15pm.
A spokesperson from BA said: "We apologised to customers and arranged a replacement aircraft to ensure they were able to continue their journey."
The pilot failed to disarm the plane door when he opened it to pass documents to ground crew. Had he done so, the emergency slide would not have deployed.
The captain was taken off flying duties immediately, while the passengers waited more than three hours before they left in another aircraft with a different pilot.
LATEST DEVELOPMENTS
Passengers were left waiting at Heathrow
PA
In December, it was announced that British Airways will fly from London Stansted from next year for the first time since before the coronavirus pandemic.
Holidaymakers will be able to fly to Florence, Italy from £41 each way and Nice, France from £43 each way. They can also grab a deal with flights to Ibiza, Spain from £36 each way.
New routes are available for destinations from both Heathrow and Gatwick including Fuerteventura, Riga and Belgrade.
Holidaymakers can also bag new flights to Aruba, Guyana, Sharm El Sheikh and Cologne.
Tourists will also be able to go from Agadir in Morocco for the first time.
CEO of BA Euroflyer Tom Stoddart said: "Agadir is a coastal destination that enjoys 300 days of sunshine per year, so we know it will see strong demand from our leisure customers and particularly those seeking holiday packages.
"It offers beaches, golf courses and water sports alongside the opportunity to experience cultural landmarks, such as Agadir Kasbah."