Shares in Boeing fell on Thursday after US regulators said they had found new flaws in the company’s grounded 737 Max models, adding to delays in bringing the plane back into service.

United Airlines and Southwest Airlines become the latest carriers to extend their bans on using the Boeing 737 Max after the US aviation regulator said it had identified a new potential risk with the plane, which has been grounded since March following two deadly crashes within five months.

After the Federal Aviation Administration (FAA) said on Wednesday that Boeing must address the new issue before the jet can return to service, United and Southwest joined American in continuing to ground the plane beyond August.

United said it would not use the plane until 3 September, while Southwest said it would wait until at least 1 October. The planes have been grounded since crashes in Indonesia and Ethiopia killed 346 people.

The latest risk was discovered during a simulator test last week but it was not yet clear if the issue can be addressed with a software upgrade or will require a more complex hardware fix, sources told Reuters.

Boeing was the biggest faller on the blue-chip Dow Jones industrial average index in early trading on Thursday. Shares fell by 2.3% to hit $366.2 – about 14% below their level shortly before the second crash on 10 March.

The FAA did not elaborate on the latest setback for Boeing, saying only that it was a “potential risk that Boeing must mitigate”.

The new issue means Boeing will not conduct a certification test flight until 8 July at the earliest, Reuters sources said, and the FAA will spend at least two to three weeks reviewing the results before deciding whether to return the plane to service.

Last month, FAA representatives told members of the aviation industry that approval of the 737 Max jets could happen as early as late June.

However, the US regulator has also come under pressure to act in concert with other regulators, after the chaotic handling of bans of the aircraft in different parts of the world.

The International Air Transport Association (IATA), which represents the bulk of the world’s airlines, called for aviation safety regulators to work together on safety standards and the timeline to reintroduce the planes to service. Members of the lobby group met on Wednesday in Montreal, Canada, to discuss their plans for the 737 Max.

Alexandre de Juniac, IATA’s director general, on Thursday said: “Aviation cannot function efficiently without […] coordinated effort, and restoring public confidence demands it.”

Boeing, the world’s largest planemaker, has been working on the upgrade for a stall-prevention system known as MCAS since a Lion Air crash in Indonesia in October, when pilots were believed to have lost a tug of war with software that repeatedly pushed the nose down.

A second deadly crash in March in Ethiopia also involved MCAS.

In a statement published on Wednesday evening, the FAA said it was following “a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service”.

The regulator said it would lift its ban on the plane flying “when we deem it is safe to do so”, amid continued testing of Boeing software changes and a new training programme.

Boeing said it was working on the software required by the FAA.

In a statement, the company said the FAA had “identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months”.

Boeing’s aircraft are being subjected to intense scrutiny and testing designed to catch flaws even after a years-long certification process for its best-selling aircraft.

The company’s chief executive said this month that the company should have been more open about the 737 Max problems.