The National Theatre will close "for an undetermined amount of time" after "Pirates of Penzance" ends its run on May 2, and "virtually all employes will be laid off," according to a letter distributed Tuesday to the approximately 100 members of the theater's staff.

The letter, written by theater manager Richard Schneider, also said that "there may be some construction activities beginning during this period," and instructed employes to clean out desks and return keys and office supplies. Affected in the layoff are office staff, stagehands, musicians, ushers, box-office personnel and telephone operators.

Although no mention was made in the letter of a reopening date, Maurice B. Tobin, president of the board of the New National Theatre Inc. (NNTI), said yesterday the theater would resume operation on Sept. 15.

Schneider described the layoff as standard procedure, prompted by the fact "that there are no bookings. The layoffs are until we have another show." However, one longtime employe of the National commented yesterday, "It's the first time we've ever gotten a letter like this."

Informed early yesterday of the layoff notice, Gerald Schoenfeld, chairman of the Shubert Organization, which books the theater, said, "I know nothing of it and have no prior knowledge of it." Recently, relations between NNTI and the Shuberts have come close to the breaking point over what the Shuberts term Tobin's "intolerable interference with effective management of the theater" and what Tobin views as a Shubert "power play to wrest control of the National."

Tobin also has claimed repeatedly that the theater is unsafe because of damage it has suffered due to nearby construction activity. That claim has been disputed by both the Shuberts and the Pennsylvania Avenue Development Corp. (PADC), which dispatched independent teams of engineers to verify conditions.

NNTI has appointed a two-man negotiating team to try to resolve its differences with the Shubert Organization, but a meeting in New York earlier this week brought the two sides no closer. The Shuberts are still pressing for the opportunity to discuss the issues, face-to-face, with the members of NNTI's board. Over the objection of some members, the NNTI executive committee flatly rejected that request last month. Tobin said he was "optimistic" that the theater would reopen in September with a Shubert-booked show, but added that there is "nothing put in writing as yet" and that another management team is "always an option."

The situation is further complicated by the construction work on an adjoining office-and-hotel complex, undertaken by Quadrangle-Marriott. Quadrangle-Marriott holds a 99-year lease on the theater and is responsible for its renovation, under the terms of its overall development contract with PADC. However, the extent of renovations and the date at which they would begin have been matters of negotiation between Quadrangle-Marriott and NNTI for more than 2 1/2 years.

The various disputes have produced a stalemate. The Shuberts claim that they cannot book shows in the future unless the construction issue is resolved and they have firm dates during which the theater will be dark. But Robert Gladstone, president of Quadrangle-Marriott, said yesterday that while his firm was "anxious to get the renovation going," NNTI has "given us no response on their desires or their date of closing."

Although the letter to the National employes indicated that construction work might begin soon, Gladstone said that he had not been apprised of that decision. He explained that with sufficient lead time, the renovations, which include expanded lobby space, elevators, new dressing rooms, a new heating and air-conditioning system and remodeled restrooms, would take "about 16-18 months." That work could be done in several stages--during summer months, for example.

Tobin envisions three of these items' (lobby, restrooms and heating system) being completed over four months this summer, and the rest of them over future summers. Gladstone, however, who termed the continuing negotiations with Tobin and the NNTI "very frustrating," said work could begin only when a comprehensive agreement on renovations and future maintenance is reached.

The National's closing contributes to a worsening employment situation locally for backstage personnel. A spokesperson for Ford's Theatre said it probably would be dark this summer. The Warner has no imminent theatrical bookings. Leveled by fire, Wolf Trap is struggling to salvage a partial season. "Right now, it looks like one of the worst summers for us," said John Ryan, president of Local 22, IATSE, the stagehands' union. Approximately 20 members of his union are affected by the National's decision to close.