The Tribune Co. announced today that it had "terminated all discussions" with former Washington Star owner Joe L. Allbritton about selling him the New York Daily News.

The sudden announcement came only two days after the Tribune Co. had extended Allbritton's option to buy the News by five days until May 5. The extra time was to give Allbritton a chance to break a deadlock with the paper's unions over his "non-negotiable" demands for a two-year wage freeze and a 5-year no strike pledge.

Until today, the Tribune Co., the Chicago-based owner of the News, had referred to Allbritton as its "buyer of last resort," suggesting to the unions that either they reach agreement on concessions with Allbritton or the News would be closed.

In a statement yesterday, Allbritton said, "The Tribune Co. has now informed us that in their opinion the likelihood of acceptable agreements with it or the unions do not appear realistically attainable."

Stanton R. Cook, president of the Tribune Co., in his brief announcement ending the deal with Allbritton, also said: "The Daily News has asked to meet with the Allied Printing Trades Council on Friday."

The cryptic release from the Chicago headquarters indicated that the Tribune may be returning responsibility for the negotiations to Daily News officials.

Last year, after the News lost $20 million in its abortive afternoon edition and with circulation and ad revenue slipping, the News' management began negotiating concessions from the paper's 11 unions and submitted a plan to Chicago calling for extensive staff cuts and other economies.

But on Dec. 18, with the News facing a loss of $11 million in 1981, the Tribune Co. announced the tabloid was for sale, turning to a New York investment banking firm to seek a buyer.

On April 1, Allbritton was announced as the prospective buyer and was given until April 30 to gain the union concessions.

But although Allbritton had promised to negotiate "personally" with the unions, he did not, leaving his interests to two young lieutenants, William Dean Singleton, a vice president of Allbritton Communications Co., and John Buzzetta, publisher of the Allbritton-owned News of Patterson, N.J.

The Allbritton team is given low grades for its preparation and performance here by virtually all observers of the talks. News executives, experienced at dealing with the New York unions, were amazed that the Allbritton team did not seek their help until it was too late.

There may be another reason for the Tribune Co.'s abrupt turnabout. It is generally believed--and not disputed by the company--that the Tribune would have to pay about $90 million to fund pensions, pay severance and other expenses if the News failed.

But the unions have threatened to sue the Tribune Co. if it allowed the News to fail without exhausting all efforts to save it. The unions would go to court to enforce lifetime job guarantees in the labor contract which runs to 1984. According to sources, Tribune Co. lawyers reportedly are concerned that the unions might win such a suit.