Fifteen cabin and flight crew staff have been arrested after 50kg (110lbs) of cannabis was found on a flight from Johannesburg to Heathrow Airport.
The haul, worth about £150,000, was seized from a South African Airways plane, customs officials said.
Officers from the UK Border Agency found the drugs in three bags after the plane landed in London at 8am.
A customs spokesman said those being held in custody will now be interviewed by investigating officers.
He said customs and UK Border Agency officials play a vital role in the fight to prevent illegal drugs from entering the UK.
He urged anyone with information about activity they suspect may be linked to drugs smuggling to call the 24-hour customs hotline on 0800 59 5000.