Two prominent museum organizations said on Tuesday they are “deeply opposed” to plans by the Berkshire Museum in Massachusetts to sell 40 works and use the proceeds for its endowment and to improve its building.

The plan to sell works by artists like Norman Rockwell, Frederic Church, Albert Bierstadt and Alexander Calder would violate the ethical codes of both organizations, the American Alliance of Museums and the Association of Art Museum Directors, which emphasize stewardship and hold that artworks should be sold only in specific circumstances to add new works or care for existing works.

“One of the most fundamental and longstanding principles of the museum field is that a collection is held in the public trust and must not be treated as a disposable financial asset,” the two groups wrote in a joint statement.

But the museum, which announced plans on Monday for a new “innovative 21st-century institution,” has said that the items it intends to auction are essentially superfluous to its plans for the future. It said in a statement that “the works that have been selected for deaccession have been deemed to be not essential to the museum’s refreshed mission,” which has a “heightened emphasis on science and history.”