Armed police boarded a flight seconds before takeoff after cabin crew called for help over fears the pilot was drunk.
Shocked passengers watched on as the armed officers rushed onto the plane at Gatwick Airport.
Cabin crew had called 999 after allegedly smelling alcohol and feared one of the officers might be under the influence.
The packed 300-passenger Boeing 777 flight 2063 was due to jet off at 8.20pm on Thursday on an 11-hour journey to Mauritius.
Police led the pilot away.
One person told the Sun: "Cops rushed onto the plane and headed straight for the cockpit.
"The first officer was cuffed and led away. A number of passengers were open-mouthed. It’s terrifying to think what might’ve happened."
The flight was delayed until 10.56pm when a replacement pilot was found.
A British Airways spokesman said the company was taking the matter extremely seriously and was helping police with their enquiries.
"We are sorry for the delay to our customers. The aircraft remained at the gate until an alternative third pilot joined the flight crew," he said.
"The safety and security of our customers and crew is always our top priority."
Sussex Police said they had been called at 8.25pm to the South Terminal Gate 17
A 49-year-old man, from Harmondsworth, West London, has been arrested.