A House committee threatened yesterday to cut off all executive branch procurement funds if President Reagan does not back away from his decision to have federal agencies ignore part of a year-old law that he deemed unconstitutional last year.

The threat by the House Government Operations Committee was contained in a report endorsed by all 23 Democrats and 13 of the 16 Republicans.

Reagan supported the Competition in Contracting Act of 1984, a bill designed to increase competition for government contracts, but in signing the bill he said he opposed a section that allowed the General Accounting Office to hold up a contract award if a bidder filed a legitimate protest.

The GAO is generally regarded as an arm of the legislative branch, but House lawyers, citing precedents, have said the agency could play a role in reviewing executive branch procurement.

Chairman Jack Brooks (D-Tex.) said "the president seems to think he has some divine right to destroy the checks and balances carefully built into the Constitution by our founding fathers."

Committee Democrats indicated that they might move to stop funding the operations of the Justice Department and the Office of Management and Budget, the two agencies ordered to implement Reagan's decision not to comply with the statute. Committee Republicans opposed that move.

Several suits have been filed against the administration for not complying with the review provisions, and the Justice Department has already lost two. Yesterday, the Justice Department appealed a decision by the U.S. District Court here to the 3rd U.S. Circuit Court of Appeals. It involves Amercon Inc., a small construction firm that submitted the low bid to clean pipes at the U.S. Military Academy at West Point but lost the contract to another firm on a technicality.