Pakistan International Airlines has been rocked by multiple scandals in recent years.

An airline is under investigation over reports it let extra passengers stand in the aisles for a flight from the Pakistani city of Karachi to Saudi Arabia.

As many as seven undocumented fliers were on board the three-hour Pakistan International Airlines flight PK743 on January 20, Pakistan's Dawn newspaper reported.

The extra passengers did not appear on the aircraft's passenger list and carried handwritten boarding passes.

It wasn't clear how the extra passengers were allowed on board, in alleged breach of air safety regulations.

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The Boeing 777 aircraft had a seating capacity of 409, but there were 416 fliers on board, Dawn reported.

The newspaper said the captain, senior cabin crew and airline were all blaming each other for the incident.

Sources told Dawn the lead cabin crew member complained to the captain about "chaos" in the cabin, but the plane was already on the taxi way and the captain did not want to turn back.

However, Captain Anwer Adil said in a statement that he was not aware the plane was overloaded until after takeoff.

"Immediate landing back at Karachi was not possible as it required lot of fuel dumping which was not in the interest of the airline."

He added that some of the passengers had asked him at the check-in counter whether they could have staff seats on the plane, and he had "categorically refused", and that no one should have been allowed on the plane without computer-generated boarding passes.

The deeply-indebted airline has been rocked by multiple scandals and a fatal crash in recent years.

Earlier this month, British fighter jets were scrambled to escort one of its planes, before a passenger was arrested on fraud charges.

In 2013, a drunk pilot was arrested in Britain, shortly before attempting to take off with more than 150 passengers on board his plane.

In November last year, a plane made an emergency landing over a blocked toilet.

One month later, 48 people were killed when one of the airline's plane crashed in Pakistan.