OTTAWA -- Prime Minister Justin Trudeau should apologize to a pair of national aboriginal organizations excluded from upcoming discussions on climate change in Vancouver, says NDP Leader Tom Mulcair.

Both the Congress of Aboriginal Peoples and the Native Women's Association of Canada have written to the prime minister and the premiers to complain about being excluded from the meetings, which start Wednesday.

Trudeau should ensure the groups are at the table, Mulcair said in an interview with The Canadian Press.

"There's really no excuse for it," he said. "It is a slight. He should apologize and just do the right thing and invite the two remaining groups."

Trudeau promised to engage with all five nationally recognized aboriginal organizations, so there is no reason to exclude two of them this week, Mulcair added.

"It is incomprehensible that he would try and pick and choose which groups get to sit at the table when he promised that all five national groups would be there," he said.

Dwight Dorey, the national chief of the Congress of Aboriginal Peoples, said he has a flight booked to Vancouver, despite the lack of an invitation.

Dorey is confused why the government paid for him to attend the COP21 climate change discussions in Paris, but now he can't have a seat at the Vancouver discussions.

"I don't see why I'd be excluded now," he said. "I'm just going (to Vancouver) with the understanding that I would expect to be there."

Trudeau has also faced similar pressure from the premiers ahead of the meeting.

Manitoba Premier Greg Selinger said all five groups should be able to attend.

"We've had that tradition in the past when they meet with the Council of the Federation," Selinger said in an interview. "We meet with all five organizations and we're supportive of continuing that practice."

In a statement released last week, the Prime Minister's Office said Trudeau would meet the premiers, the Assembly of First Nations, the Inuit Tapiriit Kanatami and the Metis National Council ahead of the first ministers meeting in the context of a renewed nation-to-nation relationship.

The meetings do not in any way preclude ongoing discussions with all five national aboriginal organizations, as committed to by the prime minister late last year, the statement said.

"The government of Canada has committed to working and meeting regularly with the national aboriginal organizations, and will continue to engage in robust bilateral discussions with all five ... on issues of importance to their members," said spokesperson Andree-Lyne Halle.

This explanation does not hold weight, Mulcair said.

"The best way we start getting to solutions is to have all five at the table ... otherwise any nation-to-nation approach risks becoming an empty shell because they've promised that they'd all be there," he said. "You can't be respectful and you can't have a nation-to-nation approach if you're deciding to exclude some."