The Clinton administration plans to ask Congress for an additional $2.2 billion over the next five years to build a system for guarding the United States against ballistic missile attack, a jump of about 20 percent over the $10.5 billion price tag projected only a year ago, defense officials said yesterday.

The administration's 2001 budget request to Congress, now in the final weeks of drafting, will reflect the increase, the officials said.

President Clinton is due to decide next summer whether to proceed with plans to deploy a limited antimissile system by 2005. But the Pentagon has begun to budget money for the program on the assumption it will receive a green light.

An independent review panel recently found the program desperately short of spare parts and suffering from insufficient test equipment. Some of the extra money would pay for test gear and more hardware for the "kill vehicle" interceptor. But the bulk of the increase is to expand the planned system to 100 interceptors, from the previously announced 20, and to upgrade five early warning radar facilities that will track enemy missiles headed toward the United States.

Confirming the increase, which was first reported by a trade industry online news service, InsideDefense.com, senior defense officials said it reflected no basic change in the program but rather a more realistic appraisal of the costs.