Embattled US aviation giant Boeing is facing another issue with its grounded 737 MAX series, as the aeroplane manufacturer works to get the aircraft series in the air again.

Foreign object debris was found in fuel tanks of some of the aircraft during maintenance work while they were being kept in storage, Boeing said on Tuesday.

This led to a comprehensive internal investigation and immediate corrective measures to the production system, according to the manufacturer.

Boeing has a backlog of 737 MAX airliners which cannot be delivered because the jet series was grounded in March 2019 after two crashes with a total of 346 fatalities.

Despite this most recent setback, Boeing still expects that the 737 MAX series will be approved by regulators and allowed to fly again in mid-2020.

The primary cause of the crashes in October 2018 and March 2019 is thought to be a fault in the flight control software, which Boeing had wanted to fix with an update more than a year ago.

Boeing is one of the largest exporters in the US and competes with Europe’s Airbus.