A citizens' group established by Congress to monitor developments at the United Nations Educational, Scientific and Cultural Organization (UNESCO) has found that the seven-month U.S. delay in spelling out changes it wants in UNESCO "greatly hampered" the organization's effort to reform itself.

In a confidential report given Nov. 27 to Secretary of State George P. Shultz and obtained yesterday by The Washington Post, a U.S. National Commission for UNESCO panel also found that major reforms have begun and cited "a clear gap between the U.S. position and the position of the vast majority of U.S. allies" on the U.S. plan to withdraw from UNESCO by Dec. 31.

President Reagan is expected to confirm the withdrawal formally within the next few days, ending 38 years of U.S. membership.

The commission of 90 private citizens and nongovernmental organizations acts as liaison among the U.S. government, UNESCO and U.S. citizens and has opposed U.S. withdrawal plans. But Shultz asked it to send observers to UNESCO's September-October governing board meetings in Paris and report to him and the commission.

The report will go today to the commission's annual meeting, which is expected to ask again that the administration reverse its withdrawal decision.

"The perceived need for reform is widely shared," the panel wrote, adding that "the director general has taken steps . . . to initiate and implement a very wide range of reforms and improvements."

At the governing board meeting, UNESCO member nations expressed "anger and frustration" because of "a general belief that no matter what efforts were made by the other nations, it would not affect the U.S. decision," the report said.

"Most of our allies believed that necessary reforms -- many of which they also seek -- can best be gained by working from within the organization," it said.

The United States first detailed proposed reforms last July, seven months after announcing withdrawal plans. The delay "greatly hampered the efforts of reform by UNESCO and by member states," the report said.

It noted that no member of the "western group" of U.S. allies supported a U.S. proposal that would have given them a veto over UNESCO's budget. It said other U.S. concerns have been addressed in freezing UNESCO's budget, listing some controversial programs for review and decentralizing UNESCO administration.