Pakistan International Airlines spokesperson Mashood Tajwar said security personnel at Benazir Bhutto International Airport seized heroin from the flight PK-785 just as it prepared to take off for London.
Five persons have been detained.
Because of the development, the flight's departure was delayed by more than two hours, the spokesperson said.
The Pakistani national carrier has attracted attention lately over seizure of narcotics on board its flights.
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The back-to-back incidents have dent the reputation of the airline. It has become a source of embarrassment for the Pakistani government and lawmakers have demanded that it be privatised.
PIA has been incurring losses running into billions of rupees and its reputation has nosedived.
The acting CEO of the airline, German national Bernd Hildenbrand, was recently stopped from going out of Pakistan, and he and two other top officials were investigated for alleged corruption in the sale of aircraft and its parts.