Attorneys for the Tennessee Valley Authority told a federal court today that the agency had made a multimillion-dollar error by incorrectly computing the amount of poisonous sulfur dioxide its largest coal-fired power plant spews into the atmosphere.

The admission, blamed on an "oversight" by agency technicans, is a major blow to the largest air pollution settlement ever reached by the Environmental Protection agency.

U.S. District Court Judge Tom Wiseman heard the agency, which has won President Carter's blessings for reshaping its anticleanup image, acknowledge that it had "miscalculated" in agreeing with the EPA that a cleanup costing $260 million was necessary at its Cumberland plant near Nashville.

Sen. Jim Sasser (D-Tenn.), a critic of the $1.2 billion settlement - covering all 12 of TVA's coal-fired plants and negotiated with EPA by TVA Chairman S. David Freeman, a former Carter energy adviser - branded the admission to victory for TVA consumers against a utility "that was over-complying" with cleanup regulations.

The admission came in a hearing on a suit brought by TVA's distributors, who are seeking to have the agreement modified on the grounds that it would result in excessive costs to their customers.

At Freeman's urging, the Carter administration earlier this year fully endorsed the settlement, saying it believed inflation concerns were secondary to public health benefits that would be achieved by cleanup at TVA, the nation's single largest source of utility-generated sulfur dioxide, a pollutant blamed for insidious impacts on human lungs.

Asked how TVA, which the Carter administration has designated as a "model utility" in solving the energy crisis, could have such an error, agency spokesman Tom Wright said, "I don't think there is any way TVA can ever explain why this happened. It's just an oversight.

Agency engineers say they spotted the error, involving methods for computing gas velocities at the plant, only a few days ago. The admission came as the distributors asked Wiseman for permission to discover internal TVA documents on how the secretly negotiated settlement with EPA was reached.

A spokesman for one of 10 environmental groups involved in the settlement of the nation's longest running air cleanup battle said the admission of the error was a "bizarre turn of events. The decree will have to be modified, but at this point we don't know to what extent."

TVA said it would attempt to secure Department of Energy and EPA funding to go ahead with cleanup at the plant. If that funding is not forthcoming, TVA spokesmen said, the agency would have to renegotiate with EPA on the $260 million cleanup effort, which was the negotiated alternative to paying that amount in civil penalties.