An object that fell from the sky and crashed through the roof of a Shanghai factory earlier this week was part of a hatchway cover from the undercarriage of a Boeing 777, China's civil aviation regulator said on Thursday.

The loss is unlikely to have endangered the flight but it "could have posed a huge threat to the air route", said the east China division of the Civil Aviation Administration.

It is still trying to find the plane that lost the hatch on Monday.

The object struck a temporary wooden building belonging to a plant in Songjiang District, causing no casualties.

The administration will warn the airline, said one its officials.