HALIFAX -- Trial lawyers from across Atlantic Canada are taking Ottawa to court to ensure the region keeps its traditional Supreme Court of Canada seat, saying Prime Minister Justin Trudeau can't change the custom without the approval of every province.

Trudeau has said regional representation among the top court's judges is important, but he has yet to commit to replacing a retired justice from Nova Scotia with someone from the same region, arguing that other forms of diversity are just as important.

The Atlantic Provinces Trial Lawyers Association said Monday it is seeking an order from the Nova Scotia Supreme Court that would require Ottawa to amend the Constitution if it wants to drop regional representation as an unwritten constitutional convention.

Such a change would require unanimous consent of the provinces, the association said.

"It raises questions of regionalism, which are very important to Atlantic Canadians and their participation in Confederation," said Ray Wagner, a Halifax lawyer and spokesman for the association.

"The problem is that we will get swallowed up by larger population areas that get appointments -- and we get forgotten and somewhat marginalized."

Without adequate regional representation, Atlantic Canada could be hurt by future Supreme Court decisions dealing with fisheries matters, employment insurance and transfer payments, he said.

As grounds for the court application, the association argues that the absence of a judge from one of the four Atlantic provinces would "fundamentally change the long-standing composition of Canada's highest court."

The Canadian Bar Association has already urged Trudeau to respect the regional appointment custom, which does not appear in the Constitution or in Canadian case law.

The convention, in place for 141 years, shouldn't be changed without invoking the Constitution's amending formula because the Supreme Court of Canada has already made a similar ruling regarding proposed changes to the Senate, Wagner said.

In April 2014, the court slammed the door on reforming or abolishing the Senate without reopening the Constitution. The high court said then-prime minister Stephen Harper's plan to impose term limits on senators and create an election process couldn't be done by the federal government alone.

Nova Scotia Premier Stephen McNeil issued a statement Monday reasserting the province's position that the rule should not be changed.

"We believe there are excellent Atlantic Canadian candidates for the seat, and look forward to the region's continued representation on the high court," the premier said.

The court challenge in Nova Scotia comes as a deadline in the appointment process is fast approaching.

Later this month, a new, non-partisan advisory board is expected to recommend to the prime minister a list of three to five candidates who are qualified, functionally bilingual and representative of the diversity of Canada.

The seven-member panel, led by former prime minister Kim Campbell, is expected to review candidates from across the country.

However, federal Justice Minister Jody Wilson-Raybould told the House of Commons the list will include two candidates from Atlantic Canada.

"Regional representation is important and will be considered in the appointments process," Wilson-Raybould said in a statement afterwards, adding that the advisory board has been asked that the regional custom be "one of the factors" taken into consideration when making its selections.

Trudeau has said his Liberal government is "folding in all sorts of different aspects to get the best possible people to sit on the Supreme Court."

Last month, Chief Justice Beverley McLachlin suggested an appointment made without regional consideration could end up being challenged in court.

"Indeed from what I see, it could be something that might someday come before us, but this is a question I think I have to leave aside," she said at the time. "I'm the last to know what will come before the court."