In a sharply worded letter sent yesterday to the officers and members of the board of the New National Theatre Inc., the New York-based Shubert Organization has charged Maurice B. Tobin, the board's president, with "intolerable interference with effective management of the National Theatre."

The letter, signed by Bernard B. Jacobs, president of the Shubert Organization, contested Tobin's recent statements to the press about the deteriorating physical condition of the National Theatre, including exploding steam pipes, jammed fire doors, falling plaster and a flooding orchestra pit. It also challenged Tobin's "unilateral" decision to close down the National for renovations--sometime after the run of "The Pirates of Penzance" ends on May 2--as a violation of the management agreement between them.

Under that five-year agreement, now in its second year, the Shubert Organization provides bookings and professional expertise for New National Theatre Inc., the nonprofit body which governs the venerable playhouse. The Shuberts pay, in addition to rent, $100,000 a year in quarterly payments to NNTI. Profits are shared on a 50-50 basis. Almost from the start, however, the relationship between the two parties has been strained.

Tobin said yesterday that some of the conditions he originally cited have since been repaired, but added that his statements were "well documented. We're not some panicky people, new to dealing with the public. We all have a burning desire to keep that theater open, but we must face up to reality. We can't keep the theater open at the customers' risk." He branded the Shubert letter as an attempt to "intimidate" the National board and qualified the May closing date as "only a possibility." A close-down date, he said, is still in negotiation between the National and Quadrangle-Marriott, the development corporation which is building an adjoining office and hotel complex and is responsible for any damages incurred to the National during construction.

"The Shuberts are very good at power plays," Tobin said. "Obviously they would love to wrest control of the National and run it themselves. They'd like nothing better than for this board to belly up."

Commenting on Tobin's remarks by telephone from New York, Jacobs said that Tobin is merely using the issue of a deteriorating theater plant to sever the existing management agreement. "Tobin's conduct shows a callous disregard for the employment of 100 people in a time of economic uncertainty," adversely affects box office, and also makes it difficult for the Shuberts to book future attractions, he said.

Tobin has long been dissatisfied with the financial terms of the management agreement with the Shuberts, which he claims do not produce sufficient revenue for the National to maintain fully its nonprofit activities--Noon at the National and a children's theater. "We made more money for the National when we ran the theater ourselves," he said. The board will meet today at 10 a.m. to consider, among other issues, "the possibility of booking the theater on its own." The Shuberts have indicated such action could lead to litigation.

In a related development last week, the Shuberts hired an independent consulting engineer to check out the physical condition of the theater. His report, while noting the existence of cracks in the walls, and a gap of "about 2-3 inches at most" between the stage floor and the asbestos fire curtain, concluded that "they do not in any way affect the safety of people entering the theater to see a performance" and saw no necessity for repairs until the neighboring construction work is completed.

The Pennsylvania Avenue Development Corp., which awarded the construction contract to Quadrangle-Marriott, conducted its own investigation of conditions at the National earlier this month. "We have no evidence that the theater is unsafe for patron use," said PADC spokesperson Geri Porter. "We see no reason not to continue bookings."

Referring to "Pirates of Penzance," which opens Monday night, Jacobs' letter to the NNTI trustees further stated that "Shubert has guaranteed the financial success of that booking and will hold you Tobin responsible for any loss we incur, as well as any loss of profits to the venture." Without such guarantees, the Shuberts say, such large-cast productions as "Evita" and "Pirates" would not play the National.

The Shubert letter is only the latest manifestation in an ever-widening rift. In the past the Shuberts have complained about Tobin's excessive use of complementary tickets and his lavish opening night parties, which Tobin defends as a necessary part of building the National's image. To date the two sides have been unable to settle on a mutually agreeable general manager who would run the theater on a day-to-day basis.