Boeing's crisis-hit 737 Max jetliner faces a new protection issue as particles has been found in the fuel tanks of several new planes which were in storage, awaiting delivery to airlines.

The head of Boeing's 737 programme has told employees that the discovery was "absolutely unacceptable", BBC reported.

A Boeing spokesman said the company did not see the issue further delaying the jet's return to service.

It comes as the 737 Max remains grounded after two fatal crashes.

The US plane maker said it discovered so-called "Foreign Object Debris" left inside the wing fuel tanks of several undelivered 737 Maxs.

A company spokesman said: "While conducting maintenance we discovered Foreign Object Debris (FOD) in undelivered 737 Max airplanes currently in storage. That finding led to a robust internal investigation and immediate corrective action in our production system."

Foreign Object Debris is a technical term that covers any substance, debris or article that isn''t part of a plane which would potentially cause damage.

The revelation is the latest in a string of problems affecting what was once Boeing''s best-selling plane.

The aircraft has been grounded by regulators around the world since March 2019 after similarities were found in the two separate 737 MAX, one belonging to Malaysian Lion Air and other to Ethiopian Airlines, crashes that resulted in killing of 346 people.

(With inputs from IANS)