Two pilots flying a commercial airliner filled with passengers were both asleep at the controls last month, leaving the jet on autopilot for an unknown amount of time, British aviation authorities said.

Authorities have not released the names of the pilots or the airline, but disclosed that the incident took place in August aboard an Airbus A330, capable of carrying up to 350 passengers, operated by a British-based airline.

The pilots came clean to authorities on their own, reporting the incident to their airline and the U.K.'s Civil Aviation Authority.

The pilots said they had received little sleep over two nights due to busy flight schedules and planned to rotate taking 20-minute naps, but they discovered they had both fallen asleep at the same time.

A pilot reported "both members of flight crew had only 5 hrs sleep in two nights due to longer duty period with insufficient opportunity to sleep," according to a report by the CAA.

The incident sheds new light on pilot fatigue and international concerns that flight crews receive insufficient sleep to perform their jobs safely and effectively.

Inside the Secret World of Tired Pilots

The U.K. has rules governing how much sleep pilots need before they are permitted to operate an aircraft, but a CAA spokesman told ABC News it was unclear how much sleep the pilots really received because of changes in time zones when flying internationally.

"Essentially, we do not know how long these 'two nights' lasted. Due to time differences these two nights might have actually been spread over only 36 hours. The pilots could have flown out of the U.K. during the night, arrived at their destination in the afternoon local time, rested for the second night … and then flown back to the U.K. the following morning, all within legal limits," said spokesman Richard Taylor.

The head of the British Airlines Pilots Association, the union representing pilots, blamed the CAA for lax regulations and called fatigue the "biggest challenge" pilots face.

"As the regulator responsible for UK flight safety the CAA has been far too complacent about the levels of tiredness among British pilots and failing to acknowledge the scale of the underreported problem," said Jim McAuslan, BALPA general secretary in a statement.

A recent investigation into a 2009 Air France disaster, in which the plane crashed into the ocean killing all 229 people on board, found the pilot had only received one hour's sleep the night before flying.