WASHINGTON — NASA does not expect to boost funding this year for the James Webb Space Telescope (JWST) as recommended by an independent panel that found that the flagship-class astronomy mission is at least 15 months behind schedule and will cost $1.5 billion more to complete than the previous estimate of $5 billion, according to NASA’s top budget official.

The panel, led by John Casani of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., said poor management led to the cost overruns, and that funding increases of $250 million would be needed in both 2011 and 2012 just to keep JWST on track for a September 2015 launch. The official NASA launch date for JWST is still June 2014.

NASA does not expect the extra $250 million needed in the current budget year to materialize, according to Elizabeth Robinson, the agency’s chief financial officer.

“We’re already a quarter into this year and we’re unlikely to be able to implement increased funding this year at the level Casani was recommending,” Robinson said in testimony before the Senate Commerce, Science and Transportation Committee Dec. 1. “So we’re doing a bottoms-up analysis trying to get a more detailed cost estimate, looking at the phasing in the budget for that program, and we’ll be presenting more details in the 2012 budget.”

Sen. Bill Nelson (D-Fla.), who chairs the panel’s science and space subcommittee, asked whether the agency would look internally for the additional funds.

Robinson said NASA will be looking at options including budget margin in its Science Mission Directorate, which is poised to take over JWST management, and other areas of the $19 billion budget authorized for the space agency in 2011.

“We’ll be looking hard at that funding profile,” she said. “We’ll just have to do an analysis throughout the entire budget of where to make those kinds of changes.”

Funding for the JWST program has long been contained within the agency’s $1.1 billion annual Astrophysics Division budget, and this has raised concerns that other astronomy missions will take hits as NASA scrambles for funds to keep the large observatory on track. But NASA now intends to pull the JWST’s portion of that budget — $445 million is requested for 2011 — out of the Astrophysics Division and have the program managed directly by the Science Mission Directorate.

“The astrophysics missions have been affected more from prior budget reductions to astrophysics than any impact from JWST,” NASA spokesman Trent Perrotto said in a Dec. 3 e-mail. “However, by raising the JWST program to the Science Mission Directorate and Agency level, more flexibility is provided.”

The Astrophysics Division is part of the Science Mission Directorate.

Perrotto said NASA has not made any changes to its 2011 budget request, which awaits action in the U.S. Congress in the coming weeks.

“NASA is putting a JWST team in place to strengthen program management, provide recommendations, and develop a budget estimate,” he said. “Any proposed changes to the budget plan will be handled as appropriate.”

Cristina Chaplain, director of acquisition and sourcing management at the U.S. Government Accountability Office, who testified at the hearing, said NASA needs more insight earlier in the development of programs to avoid massive cost overruns and schedule delays.

“NASA really needs to pay attention to the report and really examine the accountability processes it has, the oversight processes it has to make sure they’re the most effective as possible,” she said during the hearing. “In the case of James Webb we don’t have official cost estimates or a lot of information until they’ve been in development for quite a long time, years maybe.”

White House science adviser John Holdren, who also testified, said JWST is very important to scientific understanding of the universe.

“Given its importance and given the difficulty that we now understand that program is in, I can assure you that a very hard look is being taken within NASA at how those challenges are going to be addressed and ameliorated,” he said.