Dozens of environmental organizations and nuclear-power opponents are trying to stop a federal regulator from participating in votes this month as he prepares for a new leadership role with an international group.

In a letter to the Nuclear Regulatory Commission on Thursday, the Sierra Club and 33 other groups called on the panel to postpone its Aug. 26 votes until Commissioner William Magwood leaves to become director of the non-government Nuclear Energy Agency on Aug. 31.

“Mr. Magwood has created a real and apparent conflict of interest by pursuing and accepting a position with an international agency whose primary purpose is to promote nuclear power, at the same time he serves on a U.S. agency that is dedicated to protection of public health and safety and the environment,” the letter said.

Magwood wrote a letter to the groups in July challenging the notion that the Nuclear Energy Agency promotes nuclear power. He also said that he plans to participate in the upcoming votes.

“The NEA does not advocate any particular outcome, but, with the support of its member countries, focuses on facilitating policy analyses, sharing information and experience among its members, developing cooperative research projects, and developing consensus positions on technical issues, including those relevant to nuclear safety regulators around the world,” Magwood said.

The commission and Magwood declined to comment for this article.

The Aug. 26 votes concern a proposed rule on long-term nuclear-waste storage and another plan to lift a hold on reactor licensing that the commission approved two years ago.

Magwood told the groups that his participation would have “no impact on the NEA’s financial health or even its future research or policy activities.”

But the opposing groups said a vote by the commissioner would lead to questions about whether he “influenced the agency to gloss over the serious problems that attend the long-term storage and disposal of spent reactor fuel, in order to fulfill his new mission of promoting nuclear energy worldwide.”

The Nuclear Energy Agency is part of the Organization for Economic Cooperation and Development, a group that promotes economic progress and world trade. The agency’s stated mission is to “provide authoritative assessments and to forge common understandings on key issues as input to government decisions on nuclear energy policy.”

Updated Aug. 21, 9:30 p.m.: An earlier version of this article referred to the Nuclear Energy Agency in the first paragraph as an “industry group.” The description of the organization has been changed to “international group.”