The Liberals and New Democrats have declined an offer from the Privy Council Office to attend a confidential briefing just days before the federal election to learn more details of the Trans-Pacific Partnership trade agreement, saying the meeting was offered at the "last minute" and would still leave Canadians "in the dark."

Campaigning in Sherbrooke, Que., Thursday night, NDP Leader Tom Mulcair said he refused because he would be forbidden by the government from revealing the details to voters.

"Every Canadians has the right to know what is in that trade deal before Monday," he said. "And I call on Stephen Harper to release the full text of the TPP so Canadians can vote accordingly."

In an open letter released earlier in the day, Mulcair said he was asked to sign a confidentiality agreement before attending a briefing scheduled for Friday by senior bureaucrats on the trade deal.

Mulcair said Trade Minister Ed Fast broke a promise to make all details of the accord public ahead of election day.

"Instead of openness and transparency, Canadians are learning details through leaked information and the government's own self-serving promotional efforts. That's not acceptable," Mulcair said.

The Privy Council Office is the department that provides non-partisan support to the prime minister and cabinet. The Conservative campaign told CBC News the government asked the PCO to offer the briefing to the opposition parties.

But in a separate letter released Thursday, Liberal candidate John McCallum accused Prime Minister Stephen Harper of continuing a "lack of transparency" over the deal's details.

"Despite a commitment by the minister of international trade, Mr. Ed Fast, to release the text of the agreement so all Canadians can judge it on its merits before election day, media reports this week state that the details will remain secret," McCallum wrote.

"It is troubling that with just four days remaining until election day, you continue to refuse to release the text of the agreement for Canadians to see."

McCallum noted that a previous briefing attended by party representatives on Oct. 4 "provided no actual details beyond the limited information already released publicly."

"It is simply not possible to conduct a meaningful, in-depth analysis of the 1,500-plus page agreement in 90 minutes," he wrote.

I am included in briefing. I was only leader to participate in the 1st <a href="https://twitter.com/hashtag/TPP?src=hash">#TPP</a> briefing. <a href="https://twitter.com/hashtag/GPC?src=hash">#GPC</a> <a href="https://twitter.com/hashtag/elxn42?src=hash">#elxn42</a> <a href="https://twitter.com/CanadianGreens">@CanadianGreens</a> <a href="https://twitter.com/Politicolnews">@Politicolnews</a> <a href="https://twitter.com/PnPCBC">@PnPCBC</a> —@ElizabethMay

Conservative campaign spokesman Kory Teneycke told CBC News the Liberals initially agreed to attend the Friday briefing, while the NDP declined. Teneycke said the briefing was to be based on the chapter summaries, since the final text does not exist yet.

A Liberal campaign spokesman referred CBC News to McCallum's letter, but said any suggestion the party had accepted the offer of the briefing was false.

The massive trade agreement, which was reached by representatives of the Canadian government and 11 other countries in the midst of the federal election campaign, has emerged as a major campaign issue. The NDP immediately vowed to scrap the accord if elected to form a government, while Liberal Leader Justin Trudeau has said he wants to learn more about the deal.

Despite his concerns about the offer of the briefing, the Liberals' McCallum stressed in his letter the TPP "stands to remove trade barriers, widely expand free trade for Canada, and increase opportunities for our middle class."