The boss of Emirates Airline has warned Boeing is in the “last chance saloon”, saying he had seen a “progressive decline” in its performance.
Boeing has come under scrutiny after a panel on a 737 Max 9 passenger jet blew off mid-air last month.
Sir Tim Clark, who is the president of Emirates, is one of the most high-profile figures in the airline industry.
He made the comments in an interview with the Financial Times newspaper.
The BBC has contacted Boeing for its response to Sir Tim’s comments.
Last week, Boeing’s chief executive acknowledged that it faced a “serious challenge” and that the company “must do better” to win back the trust of stakeholders.
Emirates told the BBC it had nothing to add to what Sir Tim said in the interview.
“They have got to instil this safety culture which is second to none. They’ve got to get their manufacturing processes under review so there are no corners cut etc,” Sir Tim said.
“I’m sure [chief executive] Dave Calhoun and [commercial head] Stan Deal are on that… this is the last chance saloon,” he added.
The Financial Times also reported that Sir Tim was preparing to send Emirates engineers to monitor Boeing’s production lines.
He said the airline would for the first time send engineers to observe the production process of the 777 at Boeing and its supplier Spirit AeroSystems.
Emirates is one of Boeing’s biggest customers. In November it placed an order for 95 wide-body Boeing 777 and 787 jets, used for long-haul flights, valued at $52bn (£41.2bn) at list prices.
On 5 January a door plug on an Alaska Airlines 737 Max 9 blew off shortly after take-off, terrifying passengers, and forcing an emergency return to the Portland, Oregon airport.
The Federal Aviation Administration (FAA) launched an investigation of Boeing’s manufacturing process and barred the firm from expanding production of its popular 737 Max planes.
Some of the company’s biggest airline customers have also expressed concerns, noting that the issues may delay approval of new versions of the 737 Max plane, the Max 7 and Max 10, that are in the works.