Attorney General William French Smith said yesterday he had "serious reservations" about the constitutionality of a 1978 law that calls for special prosecutors to investigate government officials suspected of wrongdoing.

"In some or all of its applications, the Special Prosecutor Act appears fundamentally to contradict the principle of separation of powers erected by the Constitution," Smith wrote in a letter to Michael Davidson, chief Senate legal counsel. "The Special Prosecutor Act removes the responsibility for the eorcement of federal criminal laws and lodges it in an officer who is not appointed by, or accountable to, or save in extraordinary circumstances, removable by the attorney general or the president."

Smith said the Justice Department will continue to comply with the law until it is changed or declared unconstitutional by a federal court but left little doubt of his opinion of the law known formally as the Ethics in Government Act of 1978.

The law has been criticized recently on grounds that an official about whom suspicions are raised is left in a legal limbo with charges hanging over him until the special prosecutor completes his investigation. This was the case with Timothy Kraft, a White House aid to former president Jimmy Carter, who was forced to resign because of drug charges of which he was later absolved.

"After a careful review of the act within the Department of Justice and an analysis of its practical effect over the past few years," Smith told Davidson, "I have serious reservations concerning the constitutionality of the act."

The law says that when the attorney general is told an official of the executive branch or the president's campaign organization has violated a federal criminal law, he must investigate the charge. Within 90 days of starting the investigation, the attorney general must conclude that the charge has no basis or there is reason to appoint a special prosecutor to continue the investigation.

Passed in the wake of Watergate, the act covers about 200 top federal officials. A special prosecutor, who is named by a panel of federal judges, operates independently of the Justice Department. Many allegations have been made against federal officials since passage of the act, but special prosecutors have been named only twice to pursue the allegations.

In November, 1979, Arthur H. Christy was named a special prosecutor to investigate charges that Carter aide Hamilton Jordan had publicly used cocaine while he was White House chief of staff. Last year, Gerald Gallinghouse was named to investigate charges that Kraft used cocaine while he was Cater's appointments secretary.

Both times, the special prosecutor said there was not enough evidence to bring charges, prompting Kraft to drop a suit challenging constitutionality of the act. When Kraft dropped his suit, Smith wrote Davidson to give him Justice Department views.

Justice Department sources said that while the act is "under active review," the department has no intention of challenging its constitutionality in court, in part because the law has a "sunset" provision that will make it expire in 1983.