Is another federal election debate controversy about to erupt, with Liberal Leader Justin Trudeau set to withdraw?

The co-chairs of the Liberal Party's election campaign have written to the organizer of the Munk Debates event on Sept. 28, when the three main party leaders would focus on foreign policy, to express concern that the format isn't bilingual enough.

"We agreed in principle, under the presumption that this was a bilingual debate; however, this is clearly not a bilingual debate," wrote Dan Gagnier and Katie Telford in an open letter to organizer Rudyard Griffiths Wednesday.

The debate's current proposed format is about 20 per cent French.

Organizers say they proposed "bilingual components."

"At no time did we state that the debate would be bilingual," the Munk Debates wrote in a statement in response to the Liberals.

Liberals say Justin Trudeau's participation, alongside Conservative Leader Stephen Harper and NDP Leader Tom Mulcair, will depend on whether their "serious concerns" are addressed.

Trudeau's advisors are also concerned about the way the Munk Debates is selling tickets.

'Exclusive list'?

"It has come to our attention that the Munk Debate will be charging admission to an exclusive list of Canadians who want to attend," the letter continues. "We cannot accept that."

"The days of political leaders auditioning in front of the country's wealthiest elite are behind us. Debates should be open to all Canadians, regardless of where they live or with whom they associate."

The Liberal chairs say they believe in keeping their commitments on debates ahead of the Oct. 19 federal election, and that the focus on foreign policy is more appropriate than ever given the ongoing crisis in Syria.

"Our decision to participate in this debate will be contingent on your ability to meet these concerns in a material way."

Conservative spokesman Kory Teneycke said the Liberals' reaction is "not really a surprise."

"The Liberal campaign knows Justin 'just isn't ready' for a substantive debate on foreign policy," he wrote in an email.

Mulcair has said that he wanted there to be the same number of English and French debates overall. Making the Munk debate bilingual appeared to reach a compromise for a fifth event, the maximum number Harper would agree to.

"We firmly stand by our criteria for complete equality of French and English," NDP spokesman Brad Lavigne wrote in an email to CBC News.

At an event in Winnipeg on Thursday, Mulcair told reporters that he'd agreed to a bilingual debate and his team is working with organizers to make sure what he'd agreed to is actually the format. He said he had no indication to the contrary.

Everything else about the event was known from the beginning, he said, so "it shouldn't be an excuse for pulling out of the debate."

Tickets sold to premium members

Speaking to CBC News on Thursday morning, Griffiths confirmed that his organization submitted a draft proposal to all the parties last week and invited feedback. The Conservatives and NDP engaged in further discussions that were "constructive," he said, while the Liberals released their letter to the media.

"That's their choice, how they wish to respond," he said.

Griffiths said he hoped discussions could continue to determine a format all three would agree to, with a final set of rules and format to be nailed down next week. He didn't rule out that the amount of French may increase.

"I think that's possible. Anything is possible," he said, noting that simultaneous translation would be offered for both cable television broadcast and internet streaming.

On the ticket sales issue, however, it's less clear anything would change. Griffiths says the plan was always to sell tickets in the same manner as other debates — offering tickets to a subscribed membership first, before opening sales to the general public. Tickets sold out within hours yesterday, in an advance sale offered to premium members of the Munk Debates.

Griffiths says far from being elite, the 30,000 members of the Munk Debates represent a "cross-section" that includes students and seniors as well as professionals in the Greater Toronto Area. It's free to join as a member, although only premium members who donated $20 or more could access the pre-sale for the debate tickets.

Criticizing this group as "elites," Griffiths says, "fundamentally misunderstands who and what the Munk Debates community is."

Ticket sales cover only some of the organization's costs, he added, with its charitable foundation covering the rest.

4 more debates to go

Over the summer, Stephen Harper said he wanted to move beyond the traditional broadcasters' consortium format of two election debates, one in English and one in French. A series of complex negotiations ensued.

After the Conservatives said they'd accept up to five debate offers in this election campaign, the Munk Debates was successful with its pitch to the three major party leaders for a foreign policy-themed event. Green Party Leader Elizabeth May was not invited.

All four party leaders debated in an event hosted by Maclean's magazine on Aug. 6. The other upcoming election debates are: