A statement from the airline said flight BA422 had "experienced a technical issue on its landing approach into Valencia".
BA has apologised to passengers on board the aircraft, an Airbus A321. All passengers were evacuated safely by the crew and airport emergency services. Three passengers were taken to a local hospital as a precaution and have been discharged, according to CNN.
"The plane in which we flew London-Valencia has been set on fire!"
"We believe the pilots landed this aircraft wearing full oxygen masks and goggles, which is extremely challenging".
"The safety of our customers and crew is always our highest priority", the statement continued. "It was very surreal and scary".
Several passengers, including Lucy Brown and Dani Meroño, complained on social media that no one from British Airways said anything about the incident or explained how to evacuate.
Footage from those onboard shows the thick smoke obscuring vision so badly that it is hard to make out the seat in front.
A BBC editor, who was on the flight with her children, said that smoke filled the cabin "very quickly" about 10 minutes before its scheduled landing. People were nearly choking."Flight number BA422 is thought to have been affected - which left Heathrow Airport this afternoon".
"We are still waiting to hear what happened". "Felt like horror film". People crying. No BA representative.