All of the passengers and the crew had disembarked the plane when the incident took place and no one was injured.
After a flight of almost 12 hours and 9,670 kilometres, the door was wrenched from the fuselage as the aircraft was prepared to be towed from the stand at the airport to be parked remotely.
Images of the incident were shared on social media.
The incident occurred as the plane pushed back with the airbridge still attached to the second door on the left-hand side of the plane, known as the L2 door, which was used by passengers and crew alighting the plane.
An investigation into the cause of the incident is now under way at Cape Town International Airport.
The plane was scheduled to make the return journey to the UK as BA42 at 8.50pm, local time, on Wednesday night, but the journey was cancelled. A second service, BA56 from Johannesburg, accommodated the majority of delayed passengers.
A British Airways representative commented on the incident, saying: "We have apologised to our customers for the slight delay to their departure from Cape Town due to an issue with the original aircraft. Our customers have since been sent safely on their way to London."