One of the most influential citizen backers of the president's civil service "reforms" warns that recent White House concessions to big labor have "flawed" the bill, and could cripple the ability of managers to run their agencies.

Bernard I. Gladieux, a top official with the FDR and Truman administrations, outlined his second thoughts in a letter to the chairman of the Senate Governmental Affairs Committee. The committee is holding hearings on the bill that President Carter says is the "centerpiece" of his entire program to reorganize of his entire program to reorganize and improve the bureaucracy.

Although Gladieux holds no position with the government, his unhappiness with the turn that "reform" is taking is important. It could rally opposition to the bill, which, so far, has been well-received by the public and most politicians.

The former Budget Bureau and Commerce Department career official is part of the unofficial but very influential government-industry-management complex that retains important ties to the bureaucracy and Capitol Hill.

Gladieux was chairman of the National Civil Service League for 12 years. White House aides have sought his advice and support, and now fear his personal doubts about the reform package could have an impact on his "peer group" in and out of top government circles.

Gladieux, now with the Committee for Economic Development, testified earlier this year in favor of the original Carter plan in his capacity as a consultant to the prestigious National Academy of Public Administration. Most of the Nation's major good-government groups, the media and organizations such as the U.S. Chamber of Commerce have endorsed the original Carter reform plan.

The Carter proposal would streamline hiring and firing procedures in government, expedite appeals procedures considered "cumbersome" and put managers and top executives into a produce-or-perish corps drawn on military lines.

Now, however, as the bill is being massaged by special interest groups for additions and alterations, some of the early "purist" supporters are becoming worried about the final product.

Independent federal unions still oppose the reforms, which they say are designed to strip job protections from workers. Veterans groups make up the only other significant segment opposing the "reforms" They would abolish lifetime hiring and firing protections that military veterans now enjoy in federal civil service.

Carter officials figure they can get the bill though Congress, despite the veterans and independent unions, if they can hang on to AFL-CIO support by pleasing the big American Federation of Government Employees union.

To do that the White House has agreed to support a labor-management law as part of the reform plan and make substantive changes in appeal rights that would give federal unions more clout, and make them more attractive to employes. That is the major change that has troubled Gladieux. In his letter to Sen. Abraham Ribicoff (D-Conn.) he said:

"Particularly objectionable in the recent compromise agreements is the legitimizing of a divisive factor in employment by sharply differentiating between union members in bargaining units and the many employes who feel no such need for such affiliation and are not in such units." The compromise, he said, will permit a "tow-track appellate system concerning dismissals, or denial of automatic in-grade salary increases, and other disciplinary actions."

The compromises, Gladieux wrote, would allow persons covered by unions to elect to take their appeals to binding arbritation. Other employes would have to go to the proposed Merit System Protection Board. Gladieux feels the board should be the court of last resort for all employes and warned that if an optional appeals system is offered union-covered employes, "equal and uniform treatment of federal civil servants will come to an end."

He also believes compromises made to the AFL-CIO will make it tougher for managers to "weed out that fractional number who are incompetent or unsuitable" for government jobs.

Galdieux said he believes that the president, in his desire to win labor support for his plan to give management a stronger hand, has in fact compromised to the point where unions would call the shots. If that happened, he wrote, "the only valid criterion for public employment will be diluted by the strengthened power plays of organized government employes."

Gladieux wrote Chairman Ribicoff that he supports the "original" Carter reform plan, but will not go along with revisions that he believes would turn over the management of government to unions.

His comments are certain to bring a blast from the AFL-CIO, whose support for "reform" is based on these and yet-to-come concessions. It could also stir up conservatives, anti-union groups and others who have been nervously backing "reform" under the pledge that it would be apolitical and not give union new power.