Hundreds of Boeing's troubled 737 MAX planes and its prior generation 737 may contain improperly manufactured parts, the United States Federal Aviation Administration says.

Key points: The affected part is believed to cover 179 of Boeing's MAX planes and 133 NG aircraft worldwide

The affected part is believed to cover 179 of Boeing's MAX planes and 133 NG aircraft worldwide Boeing said it had identified a total of 41 planes with the fault, and an additional 200 planes will be inspected

Boeing said it had identified a total of 41 planes with the fault, and an additional 200 planes will be inspected The FAA has ordered the parts be quickly removed

The FAA 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.

Slats are movable panels that extend along the wing's front during take-offs and landings to provide additional lift.

The tracks guide the slats and are built into the wing, and the agency has ordered the parts be quickly replaced.

The 737 MAX, Chicago-based Boeing's best-selling jet, 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.

In a statement issued after the FAA announcement, Boeing said it had not been informed of any in-service issues related to this batch of slat tracks.

Boeing, the world's largest plane maker, said it had identified 20 737 MAX aeroplanes most likely to have the faulty parts and that airlines would check an additional 159 MAXs for the parts.

An additional 21 737 NGs most likely to have the suspect parts were identified, it said, and airlines are being advised to check an additional 112 NGs.

The NG is the third-generation 737 that the company began building in 1997.

The affected parts "may be susceptible to premature failure or cracks resulting from the improper manufacturing process", the FAA said.

The FAA said a complete failure of a leading edge slat track would not result in the loss of the aircraft, but a failed part could cause aircraft damage in flight.

It said it would issue an Airworthiness Directive to require Boeing's service actions to identify and remove the parts from service.

Operators will be required to perform this action within 10 days, but can continue to fly the planes during the 10-day period before the parts are removed.

'If it's is in the air by Christmas I'll be surprised'

In April, the plane maker said the two fatal crashes had cost it at least $1 billion as it abandoned its 2019 financial outlook, halted share buybacks and lowered production.

The company's shares have fallen by nearly 20 per cent since the Ethiopian Airlines crash in March.

Some international carriers are sceptical the plane will resume flying by August as some US airlines have suggested.

Tim Clark, president of Emirates, told reporters in Seoul that it could take six months to restore operations as other regulators re-examine the US delegation practices.

"If it is in the air by Christmas I'll be surprised — [that is] my own view," he said.

Boeing said one batch of slat tracks with specific lot numbers produced by a supplier was found to have a "potential nonconformance" and said airlines "are to replace them with new ones before returning the aeroplane to service".

The company said it was "now staging replacement parts at customer bases to help minimise aircraft downtime while the work is completed".

Boeing said once new parts were in hand, replacement work should take one to two days.

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

The FAA has said it has no timetable for ending the grounding of the plane. ( Reuters: Lindsey Wasson )

The issue was discovered following an investigation conducted by Boeing and the FAA Certificate Management Office, the FAA said.

An FAA spokesman said the issue should not delay Boeing's planned submission of a software update and training revisions, but it remains unclear when that will be submitted.

The FAA has said it has no timetable for ending the grounding of the aeroplane.

Reuters