"It's naïve to think a country would build a ballistic missile and not build countermeasures along with it," said Lisbeth Gronlund, a researcher at MIT's Security Studies Program. "I think there's some wishful thinking and some deliberate denial going on."

The $30 billion system is designed to protect the United States by using rocket-launched, heat-seeking "kill vehicles" guided by orbiting satellites to intercept ballistic missiles in flight.

U.S. intelligence estimates have said North Korea and Iran are expected to have operational ballistic missiles by 2005.

"We are like physicians warning the public against the introduction of an untested drug."

President Clinton is scheduled to make a decision this fall on deploying such a system. His decision will be based on the Pentagon's recommendation this summer on whether the NMD is technically possible.

"We are committed to giving enough technical information to the president for him to make an informed decision," said Navy Rear Adm. Craig Quigley, a Pentagon spokesman.

Two test flights have been performed using dummy target missiles fired from Vandenberg Air Force Base in California against interceptor rockets launched from Kwajalein atoll in the Marshall Islands more than 4,000 miles (6,435 kilometers) away.

The first test, launched last fall, was a partial success. The second, in January, was a failure. A third test is scheduled for June 26.

If Clinton decides to go ahead, the first phase of the system is expected to be operational in 2005 with about 100 interceptors, six ground-based radar and a few satellite-based infrared sensors. By 2010, the system would grow to 250 interceptors, 15 radar sites and a constellation of space sensors.

But the NMD, the panel's report said, fails to take into account simple "real-world" countermeasures that could be used by any country to disguise a missile's warhead -- making it next to impossible for the kill vehicle to tell the real thing from a decoy.

For example, warheads containing biological or chemical weapons could be divided into hundreds of "bomblets" atop a single missile, the report said. Once they were dispersed in space, the bomblets would spread over a 15-mile (24-kilometer) area, making it impossible for interceptors to destroy them all.

Or, the warhead could be hidden inside a thin mylar balloon, making it identical to other decoy balloons launched on the missile.

It also could be artificially cooled by liquid nitrogen inside an aluminum shell, thereby fooling heat-seeking satellites designed to locate it in space.

"This is a [research and development] program masquerading as an operational military program," Gottfried said. "It's like building a tank, but you never run it except in a parking lot."

The panel made clear it was not against missile defense on ideological grounds and said such research should continue.

Rather, "we are like physicians warning the public against the introduction of an untested drug," Gottfried said.

"I'm all for a cure that works," said Andrew Sessler, the panel's chairman and a senior scientist at Lawrence Berkeley National Laboratory. "I'm not for one that doesn't."

The panel included senior defense consultants to the U.S. government and nuclear weapons laboratories, as well as former members of the Defense Science Board and other defense advisory panels.

The scientists used physics and engineering calculations to analyze both the planned NMD system and steps that nations developing long-range missiles could take to foil the defense.