OTTAWA -- Justin Trudeau has recused himself from all matters related to the appointment of a new federal ethics watchdog since his own conduct is under investigation.

The prime minister -- who is under investigation for his vacation last Christmas on the private Bahamian island owned by the Aga Khan -- has designated government House leader Bardish Chagger to take charge of the appointment.

But opposition ethics critics are scoffing at the pretence of impartiality in the choice of the next ethics commissioner, noting that Trudeau is Chagger's boss and could fire her if she chooses someone not to the prime minister's liking.

Moreover, Chagger has been one of Trudeau's primary defenders in the House of Commons on the vacation controversy.

Current commissioner Mary Dawson has launched two investigations into Trudeau's vacation, in which he and his family used the Aga Khan's private helicopter to fly to and from Nassau.

Under the Conflict of Interest Act, the prime minister, cabinet ministers and parliamentary secretaries are prohibited from accepting free travel on charter or private aircraft without getting advance approval from the ethics commissioner.

Public office holders are also prohibited from accepting gifts from anyone who has business dealings with the government, although there are exceptions if the person is a friend.

Trudeau has said the Aga Khan, the billionaire spiritual leader of the world's Ismaili Muslims, is a family friend he has known since childhood.

Dawson's term as ethics commissioner has been extended twice by the Liberal government and is due to expire on July 8. She has said she won't seek reappointment.

NDP ethics critic Nathan Cullen said putting Chagger in charge of finding Dawson's successor is akin to asking an employee to appoint someone to investigate his or her boss.

"The conflict of interest remains," he said in an interview Monday.

"You just can't pretend that Ms. Chagger has independence from the Prime Minister's Office. Her key role is defending the prime minister, every single day on this issue, by the way."

Cullen also questioned whether Trudeau's top aides will still be involved in the choice of the next ethics commissioner -- a question put to the PMO by The Canadian Press but which was not answered.

"They like to keep control while pretending to be hands off."