On the eve of what could be the biggest trade deal in history, Tom Mulcair says the NDP would not be "bound" by any Trans-Pacific Partnership signed by the Conservatives

In a letter to International Trade Minister Ed Fast, Mulcair says the Conservatives have "no mandate" to negotiate the deal so close to the federal election and are stepping far beyond the traditional caretaker role.

If an NDP government is elected on Oct. 19, it would "not consider itself bound" to the deal, he says.

Fast is meeting with his international counterparts in Atlanta for final talks on the Trans-Pacific Partnership, a 12-country pact that would include 40 per cent of the world's economy. The agreement is expected to be a boon for Canada's overall economy, but could come at a cost to certain sectors such as the auto industry and agriculture.

Mulcair said negotiations have been carried out without transparency, leaving Canadians "in the dark" on details of what's on the table.

'Forfeited' mandate to sign deal?

"Your government forfeited a mandate to conclude negotiations on a major international trade agreement the day the election was called," Mulcair wrote. "New Democrats will continue to fight for trade agreements that are good for Canada, and stand foursquare against concessions that harm Canadian interests."

Conservative Leader Stephen Harper has promised he would only sign off on a deal that benefits Canada's entire economy, but the auto industry and supply-managed dairy sectors are worried they will be hard hit by trade concessions.

Furious dairy farmers brought their cattle, tractors and placards to Parliament Hill on Tuesday to protest any deal that could harm their industry by increasing imports of milk and other dairy products to Canada.

Dairy farmer Robbie Beck of Shawville, Que., holds onto a dairy cow as he takes part in a protest against the Trans-Pacific Partnership in front of Parliament Hill in Ottawa on Tuesday. (Sean Kilpatrick/Canadian Press)

Earlier this week, Finance Minister Joe Oliver dismissed suggestions that negotiating the deal was in violation of the so-called caretaker convention, designed to limit government activity during an election period.

"When there's a matter of importance or urgency for the government to deal with in the national interest, then it's appropriate for us to do that. And this is certainly one of those cases," he said Wednesday.

Liberal Leader Justin Trudeau has said he wants the Conservatives to be more open and transparent about what could be in the deal, but conceded it would be "unrealistic" to suspend international negotiations during the election.

Conservative Party spokesman Chris McCluskey said Parliament would have an opportunity to scrutinize the deal.

"Thomas Mulcair knows full well that any trade deal would be tabled, fully debated and voted on in the next Parliament," he said.

Any deal would need to be approved in Parliament.