AFL-CIO union leaders have warned the White House that the president's proposed civil service reform plans could go down the drain if President Carter tries to trim the amount of the October pay raise for U.S. workers and military personnel.

Union leaders were furious over a report here (and elsewhere) yesterday that said the president was planning to put a lid of between 5 percent and 5,5 percent on 1978 federal and militaru wage increases.

Opponents of the pay "cap" for the government have argued that it would be unfair - and politically unwise - for the president to put a lid on government salaries now before private industry wage data is available, and while the civil service shakeup bill is being debated in Congress.

White House economists, led by Barry Bosworth of the Council on Wage and Price Stability, believe that it is imperative that the government make a gesture so that labor and industry will agree to voluntary 1978 wage price restraints. Bosworth has recommended that the federal military pay raise, which had been forecast as between 6 percent and 6.5 percent, be trimmed by 1 percent.

more important than the half-billion dollars the 1 percent cutback would mean, advisers say, is the message it would send to the private sector that the president is concerned about inflation, and plans to hold down the $69 billion federal-military payroll.

Insiders still believe the president will make the pay-trimming statement, but now believe it will not come until next week after an advisory group on federal-military pay meets on Wednesday.

The executive council of the AFL-CIO had endorsed the president's civil service "reform" plan that would make it easier to fire "incompetents" and expead management authority to hire , assign and fire top career executives.

American Federation of Government Employes president Kennth Blaylock generally has endorsed the reform package, despite major opposition to it in his union, in return for support for a collective bargaining law.

"But if the 'cap' goes on federal salaries," a union official said, "you can kiss reform goodbye because we will then oppose it." If the AFL-CIO and AFGE withdraw their backing of the reorganization and reform plan, many members of Congress now supporting it would move over into the "no" vote column.