WASHINGTON -- A key test of an interceptor missile system designed to knock out enemy warheads will be delayed from April 27 to June 26 to give technicians more time to make sure the system works, a Pentagon official said Tuesday.

The $100 million test will be the third and last of a series of high-stakes demonstrations in space before President Clinton is to make a decision whether to deploy the futuristic National Missile Defense system.

The delay now raises doubts about whether Clinton will have enough time before his term ends to make such a decision. He was to make the call by October -- based on a Pentagon recommendation this summer -- on the system designed to protect the United States from a rogue missile attack.

The first test last fall succeeded when the kill vehicle slammed into the target at 15,000 m.p.h. (24,140 kilometers per hour), destroying it instantly. The second, on January 18, failed when a clogged coolant line caused the 120-pound (54-kilogram) kill vehicle to veer off course and miss the target altogether.

"What we're trying to prove is that we can hit [the target] and have it hit where we want," said Air Force Lt. Gen. Ronald Kadish, director of the Ballistic Missile Defense Organization.

"We're taking a strict technical approach to this process. All of us have a desire to be successful…but we're not taking this to the point where we're rushing things."

The delay was necessary, Kadish said, "to take as much time as we needed to get it right. There's no major design or redesign activity that needs to occur."

But the postponement also means the Pentagon will compress the amount of time -- from 60 days to 30 days -- that it has to review the program before recommending a course to Clinton.

Managers also will have only 85 percent of the data in hand from the third test during the month-long review slated for July, Kadish said.

The latest delay "is not unexpected for early development testing," said Lisbeth Gronlund, a physicist at the Massachusetts Institute of Technology's Security Studies Program in Cambridge. "The problem is, we're trying to make a deployment decision at this early stage. This is a fairly complicated system and it does not leave a lot of room for error."

If Clinton approves deployment, construction of a base for 100 interceptor missiles in Alaska and radar facilities in the Aleutian Islands would begin in the spring of 2001. The project would be completed by 2005 and is expected to cost about $38 billion over 20 years.