A new problem involving Boeing’s grounded 737 Max has been identified by the US Federal Aviation Administration.

The regulator said more than 300 of the new generation aircraft and previous 737 NGs may contain improperly manufactured parts.

The FAA will require these parts to be quickly replaced, Reuters reported.

The authority said up to 148 of the part known as a leading-edge slat track that were manufactured by a Boeing supplier are affected, covering 179 Max and 133 NG aircraft worldwide. The NG is the third-generation 737 that the company began building in 1997.

Slats are movable panels that extend along the wing’s front during take off and landing to provide additional lift. The tracks guide the slats and are built into the wing.

The 737 Max was grounded globally in March following a fatal Ethiopian Airlines crash after a similar Lion Air disaster in Indonesia in October. The two crashes together killed 346 people.

Boeing has yet to submit a software upgrade to the FAA as it works to get approval to end the grounding of the 737 Max.

The US manufacturerhas identified 21 737NGs “most likely” to have the parts in question wit airlines advised to check an additional 112 NGs.

A separate service bulletin will go to 737 Max operators to do inspections before the Max returns to service.

Boeing identified 20 737 Max aircraft that are “most likely” to have the parts in question. Carriers will be asked to check an additional 159 MAXs to ensure a “thorough assessment”.

Boeing said it was working with the FAA and had contacted 737 operators advising them to inspect the slat track assemblies on certain aircraft.

“One batch of slat tracks with specific lot numbers produced by a supplier was found to have a potential non-conformance,” Boeing said.

“If operators find the parts in question, they are to replace them with new ones before returning the airplane to service.

“Boeing has not been informed of any in-service issues related to this batch of slat tracks.

“Boeing is now staging replacement parts at customer bases to help minimise aircraft downtime while the work is completed.

“Once the new parts are in hand, the replacement work should take one to two days. Boeing will also issue a safety service bulletin outlining the steps to take during the inspections.”