A Pentagon attempt to develop an airborne laser to blast enemy missiles out of the sky has already cost more than twice original estimates and faces continued technical hurdles, according to a congressional audit of the struggling program.

The laser project, begun eight years ago, has become a key element in the Bush administration's drive to erect a network of defenses against ballistic missile attack.

The lead part of that network -- a system of land-based missile interceptors -- is scheduled to go on alert in Alaska later this year. It will aim at knocking down enemy warheads in space. The airborne laser is meant to complement that system by going after enemy missiles soon after launch in their "boost phase."

But the audit by the General Accounting Office cites a history of technical troubles and schedule delays that have plagued the program. The GAO report, to be released today, also calls into question the ultimate usefulness of the planned system.

The $2 billion spent on the program from inception in 1996 to 2003 is double the initial estimate, the report says, adding that more cost overruns are likely. The Pentagon plans to spend an additional $3.1 billion from fiscal 2004 through 2009.

"The cost growth occurred primarily because the program did not adequately plan for -- and could not fully anticipate -- the complexities involved in developing the system," the report says.

The weapon consists of a high-energy chemical laser mounted in a Boeing 747 jumbo jet. By focusing on a rising enemy missile, the laser is supposed to rupture the missile's casing and cause it to lose power or flight control.

Originally envisioned to defeat short- and medium-range ballistic missiles, the system's role has been expanded by the Bush administration to include targeting intercontinental missiles. Pentagon officials have heralded it as a revolutionary technology promising a major leap ahead in U.S. missile defense. But the GAO report warns that predictions of usefulness are based so far only on computerized models and simulations, not "on any demonstrated capability of the system."

The report adds that if fielded, the system will have a number of special maintenance and refueling needs that must be factored into long-term costs. But the Missile Defense Agency "has not yet determined the cost" of the weapon's "unique support systems," the report says.

"The General Accounting Office's findings are truly astounding," said Sen. Daniel K. Akaka (D-Hawaii), who requested the study along with Sen. Bill Nelson (D-Fla.). "They call into question whether or not it makes sense for Congress to continue funding this program at the requested level -- especially in this era of tight budgets."

In recent testimony to Congress and statements to reporters, Air Force Lt. Gen. Ronald T. Kadish, who heads the Missile Defense Agency, has acknowledged problems with the laser project but argued strongly for proceeding with it, given the big payoff if it works. In a restructuring of the program earlier this year, Kadish decided to delay purchase of a second aircraft and concentrate on two aspects: developing an optics system and demonstrating, in ground tests, that individual laser modules can be combined to generate a single laser beam.

"Once we accomplish those two things, I'll be able to tell you when we can do the rest," he told a group of journalists last month.