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Boeing  (BA - Get Report) shares extended declines Tuesday after The Wall Street Journal reported that ongoing disputes between U.S. and European regulators could delay the return of the planemaker's 737 MAX jet.

The Journal said European safety officials aren't satisfied with safety issues related to Boeing's plans to repair the 737 MAX's software,  which involves both flight control computers and will address concerns for the system raised by the U.S. Federal Aviation Administration in June.

Last month, the paper reported that a panel of international regulators will criticize the FAA's approval process for the 737 MAX aircraft, which remains grounded following two deadly crashes in March 2019 and November 2018 that took the lives of 346 passengers. The Journal said the panel, comprised of regulators from major economies around the world, will ask for more transparency from the FAA over its certification procedures.

Boeing shares were down 1.64% Tuesday to change hands at $370.37 each, a move that would wipe out most of its 30-day gains. 

Boeing shares have also been pressured by vows from EU officials to retaliate against planned U.S. tariffs on Airbus SE (EADSY) after the World Trade Organization ruled the France-based planemaker had received unfair subsidies from Brussels.

France's finance Minister, Bruno Le Maire, said the EU would respond to Washington's plans to slap a 10% tariff on European-made Airbus planes, as well as steeper levies of 25% on various other goods imported from the Continent.

The comments suggested possible reprisals against Boeing, whom the WTO has also said received unfair support, in the form of tax breaks from the state of Washington, and will offer tariff-focused remedies early next year.