BOEING has found potentially dangerous debris in the fuel tanks of its 737 Max aircraft, which has been grounded since last March after two fatal crashes.

An internal memo said that the company had found foreign object debris (FOD) — rags, tools, metal shavings and other materials left by workers — in a number of planes that were awaiting delivery to customers.

Mark Jenks, the general manager of the embattled 737 Max programme, said that the debris was "absolutely unacceptable" in the memo, which was seen by Reuters.

As a result, Boeing has ordered inspections of its 737 Max fleet, a matter of weeks after it was announced that the model could return to service sooner than expected.

In a statement the firm said: "Safely returning the 737 Max to service is our top priority. While conducting maintenance we discovered foreign object debris (FOD) in undelivered 737 Max planes currently in storage.

"That finding led to a robust internal investigation and immediate corrective actions in our production system. We are also inspecting all stored 737 Max airplanes at Boeing to ensure there is no FOD."

Although there has been a worldwide ban on the model since 346 people were killed in two separate crashes last year, Boeing has continued to manufacture the 737 Max in anticipation of its return to service.

The company has said its best estimate is that the aircraft will not be ungrounded until mid-2020, after endorsing simulator training for pilots before flights resume, and that regulators will determine the timing.

Last year the giant carrier was pushed to a monumental $1.98bn (£1.5bn) loss by the grounding, saying that the crisis would cost it $18.6bn overall.

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