An unsuccessful bidder for the District's cable television franchise objected yesterday to the City Council's recent decision to grant concessions to the winning cable firm and said it is exploring legal means to reopen the franchising process.

Last week, District Cablevision Inc. won most of the contract modifications the company sought to make its proposed cable system economically viable, under 90-day emergency legislation. The council plans to make a final decision on the modifications in the fall, after District Cablevision provides more information.

In a letter sent to council members yesterday, District Telecommunications Development Corp., one of three companies that competed for the city's 15-year cable franchise, said the changes made by the council amount to a new contract and render the council's original request for competitive bids a "meaningless pursuit."

"What has been lost in the rush to give DCI whatever it now says it needs to build a cable system in the District is the concept of fundamental fairness," wrote Charles Tate, a District Telecommunications official. " . . . It is only right and fair that District citizens should have the best system available at the lowest cost. That goal can only be achieved by a dispassionate analysis of the needs of the District, not a forced agreement undertaken with a gun to the council's head."

District Telecommunications officials said the firm spent $750,000 in bidding on the cable contract.

Tate asked the city to make a good-faith effort by allowing his firm to submit a new bid under the same terms given to District Cablevision.

Richard Maulsby, director of the city's cable television office, said the District believes that it has the legal right to modify its contract agreement with its cable firm and that the city is beyond the stage of discussing other cable proposals.

The agreement signed by District Cablevision last February allows the firm to seek changes in its proposed system in the event of adverse circumstances beyond the company's control. District Cablevision justified the modifications by arguing that it could not raise enough money to build the system unless the changes were made.

Tele-Communications Inc., a Denver-based company that is the nation's largest cable operator, has offered to provide enough money for District Cablevision to build the District's cable system provided the system is economically viable.

But because District Cablevision is seeking to reduce the size and scope of the cable system to make it viable, some City Council members raised concerns that the concessions under consideration could spark protests from the losing bidders.

Reginald F. Martin, an attorney and staff member for City Council member Betty Ann Kane (D-At Large), said yesterday that he had advised the council's cable committee that changes had to be made carefully to avoid legal pitfalls. "It's my legal opinion that the franchise agreement provides for modifications under some circumstances."

But Harold Jordan, an attorney with the law firm of Hudson Leftwich & Davenport and the legal adviser for District Telecommunications, said the modifications to the franchise agreement represent "radical changes."

"We are indeed looking at litigation as a distinct possibility," said Jordan. "I think it is a new contract. It doesn't look like anything we bid on . . . . I think we have a significant problem if this is the method by which contracts are awarded and discarded by the city."

Capital City Cable, the other firm that competed for the franchise, said it would be prepared to submit a new cable bid.