Captain Amir Akhtar Hashmi is accused of putting passengers' lives at risk by leaving a trainee at the controls of a packed jet to nap in a business class seat.
Hashmi allegedly slept for two-and-a-half-hours on board the Pakistan International Airlines (PIA) flight from Islamabad to London on April 26.
A passenger, who filed a complaint with the airline, took a picture of the snoozing pilot and shared the snap on social media.
Hashmi is believed to have handed over the controls of the plane, which was carrying 305 passengers, to a trainee pilot soon after takeoff.A Pakistan International Airline plane was forced to divert to London Stansted today
He was removed from flying duty and an investigation is underway, PIA spokesperson Danyal Gilani said.
According to the Dawn, PIA was initially reluctant to act against Hashmi, who is the former president of the Pakistan Air Lines Pilots Association (PALPA), "but later caved in to pressure from above".
Hashmi was in charge of operating the London-bound flight with first officer Ali Hassan Yazdani.
Another first officer, Mohammad Asad Ali, who was under training, was also in the cockpit, the Dawn reported.
A passenger noticed the captain sleeping in a business class seat and made a complaint to a member of the cabin crew.
A Pakistan International Airline plane was forced to divert to London Stansted today
In a report, air steward Nazneen Haider said: "Passenger (Seat 1 D) complained that while the captain was sleeping in business class cabin, I (the passenger) do not feel safe.
"It had been explained that two other crew members were in the cockpit but he said that he would follow the matter and write down a complaint card as well."
In June 2009, Captain Marc Dubois retired to the cabin of his Air France jet for a rest after sleeping for just one hour before the flight, leaving a junior co-pilot at the controls.
Pierre-Cedric Bonin, 32, and fellow co-pilot David Robert, 37, were unable to control the Airbus A330 after it hit a thunderstorm over the Atlantic Ocean.
The plane crashed into the ocean, killing all 228 people on board.