WASHINGTON — NASA’s James Webb Space Telescope (JWST) is expected to cost at least $1.5 billion more than current estimates and its launch will be delayed a minimum of 15 months, according to an independent review panel tapped to investigate escalating costs and management issues with the next-generation flagship astronomy mission.

U.S. Sen. Barbara Mikulski (D-Md.) called for the independent review in June to identify the root causes of cost growth and schedule delays on the JWST.

“The Webb telescope will now cost $6.5 billion, $1.5 billion more than the estimate included in NASA’s February 2010 budget request, Mikulski wrote in a Nov. 10 letter to NASA Administrator Charles Bolden after reading the Oct. 29 report. “Its launch will be delayed by over a year, from June 2014 to September 2015.”

Led by NASA’s Goddard Space Flight Center in Greenbelt, Md., the JWST is an infrared telescope with a 6.5-meter foldable mirror and a deployable sunshield the size of a tennis court. Northrop Grumman Aerospace Systems of Redondo Beach, Calif., is prime contractor. An Ariane 5 rocket provided by the European Space Agency is slated to launch the observatory to the second Lagrange point — a gravitationally stable spot 1.5 million kilometers from Earth.

In her letter, Mikulski said NASA must have a sense of urgency and frugality in correcting the JWST’s management problems and present Congress with a realistic budget for the program.

“We cannot afford to continue with business as usual in this stark fiscal situation,” she wrote.

The panel, led by John Casani, special assistant to the director of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., attributed the cost growth and schedule delays to “budgeting and program management, not technical performance,” according to the report, which characterized the JWST’s technical progress as “commendable and often excellent.”

However, the report notes that “there may be a number of low probability threats whose occurrence could cause an additional year delay in launch and a correspondingly higher cost.”

The panel recommends restructuring the JWST project office at Goddard to emphasize cost and schedule ceilings. “The flawed practice by the Project of not adequately accounting for threats in the budgeting process needs immediate correction,” the report states.

However, the report also found that “the JWST Project has invested funds wisely in advancing the necessary technologies and reducing technical risk such that the funds invested to date have not been wasted,” according to the executive summary. “The management approach, however, needs to change to focus on overall life cycle cost and a well-defined launch date.”

Bolden, in a Nov. 10 statement, said he agrees with the panel’s findings and that NASA would overhaul the program’s management structure.

“No one is more concerned about the situation we find ourselves in than I am, and that is why I am reorganizing the JWST Project at Headquarters and the Goddard Space Flight Center, and assigning a new senior manager at Headquarters to lead this important effort,” Bolden said in the statement.

The NASA chief said he is encouraged by the panel’s finding that the JWST is technically sound and that the project continues to meet its milestones.

“However, I am disappointed we have not maintained the level of cost control we strive to achieve — something the American taxpayer deserves in all of our projects,” he said. “NASA is committed to finding a sustainable path forward for the program based on realistic cost and schedule assessments.”