Pakistan International Airlines (PIA) says two of its passenger planes have been the subject of a bomb threat.
Flight PK709, a Boeing 777 carrying 347 passengers plus crew and bound for Manchester from Lahore, was forced to land at Ataturk International airport in Istanbul, Turkey.
The second plane, carrying 176 passengers, landed safely at its destination in Kuala Lumpur, Malaysia.
PIA said the threats to the jets were made in Pakistan.
A PIA spokesman, Capt Tasneem Mozaffar, told the Press Association news agency: "When you have 500 people in the air you have to take the safest option."
Some reports said the threats in Pakistan were made by calls, others that they were in an e-mail.
The crew of the UK-bound plane were flying over Bulgaria when they called the control tower at Istanbul to request permission for an emergency landing.
Turkey's state-run Anatolia news agency said the plane was taken to a remote part of the airport, where it was quickly evacuated and passengers were bussed to the terminal.
Sniffer dogs were sent in to check for explosives.
PIA said it had found no suspicious items, following the screening of passengers and baggage.
The airline said the flight had been cleared to continue its journey and was to land at 11:15 GMT in Manchester with all the passengers on board.
Later, PIA spokesman Mashood Tajwar said all 176 passengers on the Malaysia-bound flight had disembarked safely.
Security checks were also carried out on that plane and no explosives were found.