F-35 engines from United Technologies Corp. are proving so unreliable that U.S. plans to increase production of the fighter jet may be slowed, according to congressional auditors.

Data from flight tests evaluated by the Government Accountability Office show the reliability of engines from the company’s Pratt & Whitney unit is “very poor (less than half of what it should be) and has limited” progress for the F-35, the costliest U.S. weapons system, the watchdog agency said in a report sent to lawmakers this month.

The GAO cited the need to make design changes to the engines and then retrofit planes already built, along with continuing flaws in the plane’s software, in a report that warned the Defense Department’s “procurement plan may not be affordable.” The military plans to spend $391.1 billion for a fleet of 2,443 planes from prime contractor Lockheed Martin Corp. […]