The “nasty smell” filled the cabin which was carrying holidaymakers to Heraklion airport yesterday morning.
The pilot turned the aircraft back as a precaution and the flight was met by fire crews when it landed 90 minutes later.
Stuart Barnes, 40, told The Sun: “We were four rows from the back and the whole area stank. Everyone was saying it was cannabis.
“We were told the air con would clear it but it got even stronger.”
Passengers were eventually put on a rescheduled flight at midday.
A BA spokeswoman said no evidence of cannabis or anybody smoking cannabis was found on board.
She added: “Our pilot returned the aircraft to Gatwick as a precaution following reports of an unidentified strong smell in the cabin.
“We are sorry for the delay to our customers’ journeys.”