OTTAWA — Last Christmas, Prime Minister Justin Trudeau of Canada and his family were basking in the Caribbean sun on a private island owned by the Aga Khan, the billionaire philanthropist and spiritual leader of Ismaili Muslims.

But after an adverse ruling on Wednesday about the vacation from Canada’s conflict-of-interest and ethics commissioner, Mr. Trudeau said he would spend this holiday season at an official residence a short drive from Ottawa and in the Canadian Rockies.

In a 66-page report, Mary Dawson, the commissioner, said that Mr. Trudeau’s 2016 vacation — and two other trips made by him or members of his family — broke four sections of Canada’s conflict-of-interest law for cabinet ministers, their staff members, senior public servants and others. While Ms. Dawson imposed no penalty on the prime minister, her finding takes the luster off Mr. Trudeau’s longstanding promise to run a government beyond reproach.

Ms. Dawson found that Mr. Trudeau was not a personal friend of the Aga Khan, not in the legal sense, and should not have accepted the holiday. “Mr. Trudeau must ensure that he has arranged his private affairs so that they are not incompatible with his public duties,” Ms. Dawson wrote.