A Vancouver lawyer who has filed a constitutional challenge over prolonged Senate vacancies is willing to drop the suit if Prime Minister Justin Trudeau agrees to set clear timelines for filling empty seats in the upper house.

Ideally, Aniz Alani wants the prime minister to pass legislation stipulating that Senate seats be filled within six months of falling vacant.

But at a minimum, he wants Trudeau to make it clear that he does not agree with predecessor Stephen Harper's view that the prime minister has unfettered discretion to fill Senate seats when – or if – he chooses.

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If Trudeau does that and sets out a timeline for filling vacancies, Alani says in a letter to the prime minister that he's willing to save taxpayers the expense of a continued court challenge.

Alani filed his case 14 months ago, after Harper expressed no interest in filling the 16 vacancies which had piled up since he last appointed a senator in March 2013.

By the time Trudeau took office last fall, the number of vacancies in the 105-seat chamber had risen to 22; another two seats have fallen vacant since then while Trudeau's fledgling government is in the process of launching a new arm's-length, merit-based process for appointing non-partisan senators.

"Never since Confederation has there been as many empty seats as exists today," Alani says in his letter to Trudeau.

"While most of those vacancies accumulated before you took office as prime minister, the fact remains that the level of representation guaranteed by the Constitution has worsened, not improved, during your watch."

Alani acknowledges that Trudeau is committed to filling Senate seats, based on the recommendations of a newly created, independent, Senate advisory board. The board is expected to soon recommend nominees to fill five vacancies, with the remainder to be filled over the course of the year.

While he applauds the new process, Alani says it's still "an experiment in its very early stages" and he notes that the government seems to be proceeding "cautiously," leaving a number of provinces with considerably less than their constitutionally guaranteed representation in the Senate.

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He asks that Trudeau publicly explain whether he agrees that the Constitution requires Senate vacancies to be filled within a reasonable time – the same declaration Alani is seeking from the courts.

"Waiting for the courts to consider weighing in is not the only option and it's certainly not the most cost-effective option," Alani says in the letter.

"As prime minister, you are uniquely positioned to set standards for when Senate vacancies will be filled now and in the future."