Pakistani authorities found 20 kg (44 lb) of heroin on a Pakistan International Airlines (PIA) flight bound for London on Monday during a search at Islamabad airport, a spokesman for the national carrier said.

Heroin has been found on PIA flights at least three times since August last year, including last week at London's Heathrow Airport.

Airline spokesman Mashhood Tajwar said in a statement authorities had planned on Monday to search four aircraft.

"From one of the aircraft destined for Islamabad-London PK785, 20 kg heroin was recovered," he said.

After the search, the flight was cleared for takeoff and further investigations were going on, Tajwar said.

"PIA has taken a stern action to keep its fleet free from any effort of smuggling narcotics or other contraband," he said.

In December, Pakistani authorities seized 16.7 kg (39 lb) of heroin on board a Saudi Arabia-bound PIA aircraft.

Authorities in August arrested 12 PIA employees following the discovery of 6 kg (13 lb) of heroin on a Dubai-bound PIA plane.

On Monday last week, authorities at London's Heathrow Airport said they found "a quantity of heroin" on a flight from Pakistan "concealed within the aircraft".

They said no arrests had been made and an investigation had been launched.

"We do expect that investigation to have an international element," a British National Crime Agency spokesman said.

PIA confirmed its aircraft had been searched in London but said it had not been informed about a seizure of drugs.

The loss-making state carrier has been struggling for years.

Its chairman resigned in December less than a week after a PIA plane crashed on a domestic flight killing all 47 people on board.

In March, Pakistani authorities barred its new chief executive, German Bernd Hildenbrand, from leaving the country due to an investigation into suspected corruption in connection with the leasing of planes.

Hildenbrand, tasked with stemming losses so the government can privatize the carrier, said at the time there appeared to be confusion within the government about how aircraft leasing works. He denied any corruption.

In February, PIA investigated crew members for allowing extra passengers to travel on a flight.

At the time, the airline denied a newspaper report that seven passengers were forced to stand on the three-hour journey from Karachi to Medina, Saudi Arabia. But it confirmed that a pilot and two crew members were being disciplined.