The Philadelphia Orchestra's annual six-concert series here, an important musical fixture of this city since 1902, will be suspended after the current season, an orchestra spokesman confirmed yesterday.

Letters will be sent out to Washington patrons later this week announcing the action by the Philadelphia Orchestra, which would be high on just about any expert's list of the world's finest.

The cancellation of the Washington series is to be announced at a press conference Jan. 29 in Philadelphia during which music director Riccardo Muti will outline the orchestra's next season.

Like many other orchestras from around the world, however, the Philadelphia Orchestra will give one program here next year, under the sponsorship of the Washington Performing Arts Society.

At 84 years, the Philadelphia Orchestra series is the oldest and one of the grandest musical traditions in this city. Until the founding of the National Symphony Orchestra in 1931, those concerts were the main orchestral events available to Washington concertgoers. In those days, when concerts were more openly social events than now, hostesses would divide up into the Philadelphia Orchestra crowd and the National Symphony crowd -- with the former conferring the greater prestige.

The number of concerts offered varied over the years, but there have been six a season since the Philadelphia moved to the Kennedy Center in 1971.

Confirming news of the series' suspension yesterday, Judith Karp, the orchestra's director of public relations, said that one reason is "costs, which continue to rise. We would have to charge three times what we do to make these concerts pay for themselves." The Washington series almost sells out by subscription alone each year, and there seldom is an empty seat by concert time. Similarly unspecified "costs" were cited last year for the Metropolitan Opera's decision to abandon its national tour, including its annual visits here.

Another reason cited by Karp was the need for more concert dates for the audiences in Philadelphia. The dates Washington is to lose will be made into a new six-concert series at Philadelphia's Academy of Music.

"We have not been giving the Philadelphia market as many concerts as the demand. Subscription renewals are very high. And what remains are single seats that are not in the best spots. We now have waiting lists. And the people on them, especially the young, are not being taken care of. We cannot simply add more dates out of nowhere because we are now giving as many concerts as we can under the contracts," she said.

In the early 1980s, the Philadelphia ended its annual appearances in Baltimore, Wilmington, Del., and Ann Arbor, Mich. -- though it will return to Wilmington once next year.

The orchestra's regular series in New York will continue.