Opposition party leaders have joined forces to press Prime Minister Justin Trudeau to call byelections in all four vacant federal ridings.

Four leaders — the Conservatives’ Andrew Scheer, New Democrats’ Jagmeet Singh, Greens’ Elizabeth May and the Bloc Quebecois’ Mario Beaulieu have sent a joint letter to Trudeau decrying his decision to call a byelection in just one of the four ridings.

They say his decision denies representation for more than 330,000 Canadians in the British Columbia riding of Burnaby South — where Singh plans to run — the Montreal riding of Outremont and York-Simcoe in Ontario.

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They call on Trudeau to do what’s best for constituents in those ridings and immediately call byelections.

On Sunday, Trudeau called a byelection for Dec. 3 in the eastern Ontario riding of Leeds-Grenville-Thousand Islands and Rideau Lakes, which has been vacant almost six months since the death of Conservative MP Gordon Brown.

The prime minister must call a byelection within six months of an MP leaving a seat open.

Trudeau has said the other three ridings have been vacant for “mere weeks” and that he’ll call byelections for them in due course. Two of those remaining seats were formerly held by New Democrats and one by a Conservative.