The F-35 Joint Strike Fighter won’t fly to the Paris Air Show this summer, a development that came the same week auditors warned the program was at risk of yet another delay.

The lack of an invitation from the French for the fifth-generation fighter to make an appearance in June at Le Bourget airfield outside Paris was first reported Wednesday by Aviation Week.

The news, however, didn’t surprise at least one defense official who noted that France doesn’t intend to buy the stealthy single-engine fighter made by Lockheed Martin Corp. — and that the F-22 Raptor, a twin-engine stealth fighter, has also never turned up at the show. The official, who spoke to Military.com on background, said not receiving an invite from the French was “not a surprise,” even though F-35 leaders have attended the show in previous years.

However, the organizing committee behind the Paris air show, SIAE, told Defense News on Thursday the country did send a formal request for the F-35 to conduct a flight demonstration during the show. The U.S. defense official maintained to Military.com he was unaware of such request.

Meanwhile, in a report released Monday, the Government Accountability Office, the investigative arm of Congress, said developmental testing on the aircraft “could take an additional 12 months,” delaying the program and increasing costs by $1.7 billion.

Britain vs. France

The F-35B last year participated for the first time in the biennial Farnborough International Airshow, which features both civilian and military aircraft outside London; and the F-35A participated at the Royal International Air Tattoo, a military airshow known as RIAT held at the Royal Air Force Fairford base in Gloucestershire.

The show marked the first trans-Atlantic flight for the Air Force‘s variant just two years after its planned debut at Farnborough in 2014 was canceled following an engine fire at Eglin Air Force Base, Florida, which forced the service to temporarily ground the fleet.

The United Kingdom is a customer for the Joint Strike Fighter, with plans to buy 138 of the aircraft. But France has invested in its own Dassault Rafale and is pitching to international customers such as Canada.

While the F-35 may also skip RIAT this year, the plane is already seeing flying time in Europe. Earlier this month, the F-35A for the first time deployed to England for military drills with NATO and allied partners. It arrived this week at Amari Air Base in Estonia.

GAO: Software Testing Could Delay Program

The potential program delay and corresponding cost increase stems in part from testing required of the F-35’s software, according to auditors.

“Because of problems with the mission systems software, known as Block 3F, program officials optimistically estimate that the program will need an additional 5 months to complete developmental testing,” the GAO said. But based on the agency’s analysis of previous F-35 flight test data, “that developmental testing could take an additional 12 months.”

Block 3F software is the final “baseline” software package, designed to support a suite of internal and external weapons, including the GPS-guided Joint Direct Attack Munition, laser-guided Paveway II bomb, Advanced Medium-Range Air-to-Air Missile, and infrared Sidewinder missile. A future iteration, Block 4, is designed to integrate additional software and hardware components, including more weapons.

The report continued, “These delays could affect the start of the F-35’s initial operational test and evaluation, postpone the Navy‘s initial operational capability, and delay the program’s full-rate production decision, currently planned for April 2019.”

Pentagon Rejects GAO Assessment

The Pentagon’s Joint Program Office refuted the agency’s work shortly after the report came out. “We do not agree with the GAO’s assessment that an additional billion dollars will be needed to complete the [System Development and Demonstration]” phase, said outgoing JPO director Air Force Lt. Gen. Chris Bogdan.

“The remaining cost to complete the F-35’s $55 billion development program is estimated to be $2.3 billion — money which was already budgeted for the program,” Bogdan said in a statement.

“If there is a delay to the completion of SDD, the department has directed the JPO to maintain the resources necessary to continue SDD flight testing to May 2018.”

GAO suggested JPO not move forward with sending out a request for proposal for Block 4 until Block 3F is fully tested and completed.

Bogdan argued that doing so would “introduce undue delay” in the program, and would be “detrimental to the warfighter” and “decrease mission lethality and decrease pilot survivability” by preventing them from learning the systems sooner.

The F-35 is expected to cost nearly $400 billion in development and procurement costs to field a fleet of 2,457 single-engine fighters — and some $1.5 trillion in lifetime sustainment costs, according to Pentagon figures. It’s the Pentagon’s single most expensive acquisition effort.

Editor’s note: This story has been updated to reflect new developments from the Paris air show organizers.