Boeing’s troubled 737 MAX fleet, once grounded globally for 20 months following two fatal crashes, has been placed under a safety directive by US regulators over fire-related concerns.

The Federal Aviation Administration (FAA) said all Boeing MAX planes and some other 737 models were affected by the precautionary measure.

The watchdog – itself hugely criticised last year for its oversight of the MAX fleet in its development stages – said on Thursday that the aircraft may have a failed electronic flow control in the air conditioning packs that vent air into the hold from other areas of the plane.

The directive prohibits operators from transporting cargo in the cargo hold if airplanes are operating with this condition unless they can verify items are non-flammable and non-combustible.

The airworthiness directive impacts 663 airplanes registered in the United States and approximately 2,204 worldwide, the FAA said.

It potentially means that operators would be unable or struggle to take passenger baggage in the hold at the height of the holiday season.

It represents a new setback for Boeing as it looks to build confidence in the more fuel-efficient planes and recover from the dual-hit of the COVID-19 crisis slump in aviation.

All versions of the 737 MAX were grounded worldwide in March 2019 – days after an Ethiopian Airlines plane came down outside Addis Ababa, and five months after a Lion Air flight suffered the same fate off Indonesia.

A total of 346 people died.

The MAX fleet was only cleared to fly again in November last year following rigorous testing of new flight control systems and other adjustments.

The planes have been championed by Ryanair as “gamechanger” aircraft.

After taking delivery of its first of 210 MAX planes, the no-frills carrier said last month it was hiring 2,000 pilots to operate its fleet.

At that time, it expected to be flying 13 of the aircraft over the current summer season.

Sky News has contacted Ryanair for information on whether it has used the MAX and whether the directive will alter its plans.

Such regulations are usually followed by watchdogs globally.

TUI, which was the only operator of a MAX variant ahead of the crashes, resumed their use at UK airports in April.

The company has also been approached for comment while Boeing was yet to comment.

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