(Reuters) – A new U.S. Defense Department report warns that ongoing software, maintenance and reliability problems with Lockheed Martin Corp’s F-35 stealth fighter could delay the Marine Corps’ plans to start using its F-35 jets by mid-2015.

The latest report by the Pentagon’s chief weapons tester, Michael Gilmore, provides a detailed critique of the F-35’s technical challenges, and focuses heavily on what it calls the “unacceptable” performance of the plane’s software, according to a 25-page draft obtained by Reuters.

The report forecast a possible 13-month delay in completing testing of the Block 2B software needed for the Marine Corps to clear the jets for initial combat use next year, a key priority given the high cost of maintaining current aging warplanes.

(Reporting by Andrea Shalal-Esa; Editing by Doina Chiacu and Sandra Maler)