The Reagan administration said yesterday that it will oppose any moves by American Telephone & Telegraph Co. into data-processing or any other unregulated businesses.

In a court document opposing an AT&T request for a "clarification" of the government's 1956 consent decree with the Bell System, the Justice Department said it will oppose any effort by AT&T to enter new fields unless the phone company is restructured to prevent possible anticompetitive practices.

"Unless such structural reform is achieved, it is the position of the government that AT&T must continue to be restricted to offering communications services and facilities only where the public is protected by actual public regulation of AT&T's charges," the Justice Department said.

The policy decision sets up a possible confrontation between the administration on one side and the Federal Communications Commission and key Republican members of Congress on the other. The FCC clearly has endorsed the deregulation of vital AT&T new services, while Republican Senate members are on the verge of introducing legislation that would write the thrust of that policy into law.

Government policy concerning AT&T's structure has been the subject of disagreements between the Justic and Commerce departments during both the Ford and Carter administrations, with Justice sticking with a strict interpretation of the consent decree and Commerce endorsing the separate-subsidiary concept.

Justice's response also puts the administration's new antitrust chief, William Baxter, formally on record as endorsing the historic department position, perhaps the most important decision the former law professor has made since assuming the post last month.

The company's Washington spokesman, Pickard Wagner, said the government's position was not surprising because it is consistent with earlier Justice Department statements.

But Wagner noted that the divergence of opinions between the government agencies is "precisely why we have sought a clarification from the court." Wagner said that further delay in implementing the FCC's Computer II decision makes the March 1 deadline "all that much more difficult, if not impossible, to meet."

Last month AT&T asked U.S. District Court Judge Vincent Biunno for an interpretation of the decree in light of the FCC's Computer II decision, which ordered AT&T to set up a separate subsidiary by March 1, 1982, to offer unregulated services such as data-processing. The terms of the consent decree, however, bar AT&T from providing any services without a regulatory tariff.

In light of the FCC action and the massive restructuring of the company that the decision mandates, AT&T asked Biunno for the ruling. Since the AT&T filing, 11 companines and organizations, ranging from cable television to computer interests, have asked for intervenor status in the proceeding in light of the significant competitive consequences of AT&T's entrance into the new territory. Arguments on at least the intervention question are expected to be held on April 13.

Although the Justice Department in its memo filed with Biunno opposed granting that status, the government did say it would oppose AT&T's request and thus would represent the interests of the Bell competitors. The government said that legal precedent would not permit granting formal intervenor status and could "cause confusion and risk inconsistent resolution" of issues also being litigated in the government's ongoing antitrust suit against AT&T before a federal judge.