"We judge that, in the absence of realistic and successful testing, declaring the system operational -- and any further deployment of GMD components -- would be technically indefensible," the letter said.

Among the 22 signers of the letter were nine Nobel laureates in physics, as well as physics professors from leading US universities and research centers.

They noted that senior administration officials have said the ground-based missile defense system, which is designed to intercept and destroy incoming long-range missiles, would be 90 percent effective against a North Korean missile.

"These statements are attractive but wrong," the letter said.

"As scientists and engineers, many with long experience in advising the government on military issues, we conclude that this missile defense system will have essentially no capability to defend against a real missile attack," the scientists added.

Asserting that the system does have utility when it does not, they said, "is dangerous and could contribute to unwise decisions by US policymakers."

They said that all flight intercept tests have been highly scripted and added that until operationally realistic tests are conducted, there is no data on which to assess how it effective it would be in a real attack.

Even if the system worked perfectly, it could not defend against a missile with even the unsophisticated counter-measures available to North Korea, they asserted.

"For these reasons, we urge you to eliminate all funding to purchase or deploy any additional interceptor missiles until operationally realistic tests of the system demonstrate that it would work against a real-world attack," they said.