The US Government Accountability Office or GAO said delays in the testing of vital software for the F-35 jet of Lockheed Martin Corp are also putting the costliest weapon of the Pentagon to date in danger of being behind schedule and increasing its development expenses, Bloomberg News reported.

The congressional investigation showed that testing was experiencing a setback because of "persistent software problems." Testing is carried out to showcase the F-35's navigation, fighting, firing and reconnaissance systems. The Marine Corps want their F-35 to be combat-ready by the middle of next year. However, tests on some of its software may not be done as scheduled and may even be late by 13 months, the report said.

Bloomberg News got a draft of the report of the GAO which said, "Delays of this magnitude would mean that the Marine Corps will not likely have all of the capabilities it expects in July 2015. The effects of these delays compound as they also put the timely delivery of Air Force and Navy initial operating capabilities at risk."

The F-35 version for the Air Force is scheduled to be ready by 2016 while the Navy version has a deadline set for 2018. Officials of Lockheed Martin have not yet seen the report but in an email statement sent to Bloomberg, company spokeswoman Laura Siebert said that the contractor is "confident we will complete flight testing of the software required for Marine Corps initial operational capability this year." She added that Lockheed intends to roll out the Marine version software before July 2015, the report said.

The GAO said that after a major overhaul was done to the program in March 2012, the "acquisition cost and schedule estimates have remained relatively stable, and progress has been made in key areas." The cost of the F-35 program is pegged to be $391.2 billion, the report said.