Warning "opera is in for a difficult financial time," Washington Opera officials announced yesterday a shortened season for 1985-1986.

The number of performances in the Kennedy Center's Opera House and Terrace Theater will be reduced by 25 from the past season -- from 72 to 47. And there will be only five productions, instead of this season's seven. Four of them, however, will be new ones.

At a press conference at the Kennedy Center, officials said the company's budget will drop to $5 million from this year's $5.4 million.

The cuts were attributed by Washington Opera president David Lloyd Kreeger to increased expenses, to cuts in the company's federal grants from $500,000 to $343,000 and to the possible impact of proposed changes in the tax laws that would affect the deduction of some gifts to nonprofit institutions.

Kreeger described the tax proposals as "reckless" and "economy at the expense of the arts." He estimated that, if enacted, the Reagan administration's tax plan could result in a loss of as much as 25 percent in contributions. "These proposals may produce more dollars," he said, "but the damage they would cause to the fabric of our society would be incalculable."

Of the Opera's budget cuts, Kreeger commented, "This retrenchment we regard as temporary. For five years under Martin Feinstein the company's general director we have had a course of rapid growth, and we hope that we can resume that growth in 1986-1987."

Another factor in the company's budget problems, Feinstein said, is the size of the Kennedy Center's Opera House and the Terrace Theater -- smaller than those used by most opera companies. This means, he said, that the Washington Opera operates with a built-in deficit of 10 percent because box-office ticket sales provide only 40 percent of the Washington Opera's income, while the figure for many other companies is 50 percent. Ticket prices for the '85-'86 season will remain the same, however. Of last season's seven productions, attendance averaged 91 percent on the five highest, Feinstein said.

The announcement adds to the list of performing arts institutions here that have suffered financial setbacks, ranging from cutting in half the Metropolitan Opera's Washington season to the possible demise of the Folger Theatre. Ironically, officials said that for the first time in its 29-year history the Washington Opera's box-office gross passed the $2 million mark in its recent season, which ended a month ago.

The 1985-1986 season is scheduled to open Oct. 26 with a new production of Mozart's "Don Giovanni," conducted by Daniel Barenboim and designed and directed by Jean-Pierre Ponnelle. It is the third of a series of Mozart productions created by this team, jointly sponsored by the Washington Opera and the Orchestre de Paris. Noted baritone Renato Bruson will perform the title role. Other cast members will include Karen Huffstodt as Donna Anna, Karita Mattila as Donna Elvira, Philip Langridge as Don Ottavio, Claudio Desderi as Leporello and Faith Esham as Zerlina.

A new version of Tchaikovsky's "Eugene Onegin" will receive its premiere on Nov. 2 in a production from Florence's Teatro Comunale. It will be directed by Gian Carlo Menotti, designed by Pier Luigi Samaritani and conducted by Maxim Shostakovich. J. Patrick Raftery will take the title role. Other cast members will be Hei-Kyung Hong as Tatyana, Cynthia Munzer as Olga, Jerry Hadley as Lenski and Eric Halfvarson as Prince Gremin.

On Nov. 9 the Opera will revive its 1980-1981 production of Verdi's "Un Ballo in Maschera." Sylvia Sass will make her Washington debut as Amelia. Other performers will include Franco Bonisolli as Riccardo, Juan Pons as Renato and Geraldine Decker as Ulrica. Cal Stewart Kellogg is to conduct. Each of these productions will be in the Opera House and will have English surtitles.

There will two operas in the Terrace Theater instead of the usual four, beginning Dec. 28 with a new production of Donizetti's comedy, "The Daughter of the Regiment," in English. Joseph Rescigno will conduct, Leon Major will direct and Zack Brown will be the designer. Erie Mills will be Marie and Franc,ois Loup will be Sulpice.

Finally, beginning Jan. 4, 1986, there will be a co-production with the Opera Theater of St. Louis of Jacques Offenbach's "Christopher Columbus," a pastiche in English of some of the composer's least-known operettas. Zack Brown will design and Randolph Mauldin will conduct. David Eisler will be in the title role, joined by Robert Orth, Karen Hunt, Dana Krueger and John Fiorito.

The season will end on Feb. 2. The company has accepted an invitation to visit Jerusalem's Israel Festival in May, taking along the production of Menotti's "The Telephone" and "The Medium" that it took to the Edinburgh Festival last summer. The cost of the trip will be borne by the Israel Festival.