The White House has informed Congress it intends to cancel $4.9 billion that lawmakers approved for foreign aid programs, invoking a little-known and legally untested power to slash spending without their approval.

The 15-page notification, sent to Congress on Thursday night and reviewed by The New York Times, is the administration’s first effort to push through what is known as a “pocket rescission.” It is an effort to unilaterally claw back money that has already been appropriated by waiting so late in the fiscal year to make the request that lawmakers do not have time to reject it before the funding expires.

The fiscal year ends on Sept. 30, before the 45-day period in which Congress is required to consider a rescission request from the White House. Republicans could bring the matter to a vote sooner, but party leaders have shown little appetite for resisting the president’s spending demands and asserting their own prerogatives.

The move, the latest chapter in an intensive fight between Mr. Trump and Congress over spending powers, drew swift condemnation from the top Republican on the Appropriations Committee, who called it illegal.

“Any effort to rescind appropriated funds without congressional approval is a clear violation of the law,” Senator Susan Collins, Republican of Maine and the chairwoman of the Appropriations Committee, said in a statement on Friday.

“Given that this package was sent to Congress very close to the end of the fiscal year when the funds are scheduled to expire, this is an apparent attempt to rescind appropriated funds without congressional approval,” Ms. Collins said.