Boeing is once again pushing back the time frame for when it expects to have the 737 Max recertified for flight as the coronavirus pandemic hinders progress. Sources familiar with the latest expectations for fixing the Max tell CNBC that the plane is unlikely to be given the green light until late summer.

That is a slight, but notable, shift from Boeing's earlier plan to have the beleaguered plane cleared by the Federal Aviation Administration in the middle of this year.

Boeing still needs to complete two software updates and clear a number of other hurdles, including a recertification flight, before the Max can be cleared to return to commercial service. While Boeing has completed a number of steps to get the plane back in the air, the company says the coronavirus is slowing down work on fixing the Max.

"The Covid-19 crisis is complicating the process, but we're making good progress and focused on managing scheduling risks as they arise. All of course, subject to ongoing regulator oversight," a Boeing spokesperson told CNBC.

The Wall Street Journal first reported the slide in the Max timeline.

The 737 Max has been grounded since March 2019, following two crashes that killed a total of 346 people. Boeing executives have repeatedly set target dates for the plane's return, only to push them back as regulators detected new issues that needed to be fixed.

CEO Dave Calhoun in January said the plane would not be recertified until the middle of 2020. That timeline came after the company suspended production of the Max in December.

Calhoun has maintained in recent months that Boeing engineers are making progress clearing up issues with the Max. His strategy follows a more aggressive timeline from then-CEO Dennis Muilenburg, who was ousted at the end of last year.

The company is scheduled to hold its annual shareholders meeting Monday.