​At least 25 multi-millionaire Canadians used an offshore "sham" set up by accounting firm KPMG, a document filed Tuesday in Federal Court shows.

For more than two years, KPMG has been fighting a court order to provide the list of names of multi-millionaire clients who had used what the CRA has alleged in court documents is a "sham" Isle of Man tax avoidance structure. The court file, which had seen virtually no activity for much of that time, had remained mysteriously stalled.

Last July, KPMG lawyers told the court they were having confidential discussions with the Department of Justice on behalf of the minister of national revenue to settle the matter out of court.

Those talks have broken down, according to Mahmud Jamal, a lawyer for KPMG.

"It has been determined that a hearing will be necessary with respect to this matter," Jamal informed the court in a letter dated Monday and filed Tuesday.

CBC News and Ici Radio-Canada's Enquête first broke the story of the KPMG Isle of Man tax scheme three weeks ago, after obtaining court documents alleging that the accounting firm had "intended to deceive" tax authorities by helping multi-millionaire clients hide money offshore.

Until now, KPMG had refused to answer any questions about how many "high net worth" Canadians used the secret tax dodge that it first began marketing to wealthy clients back in 1999, promising offshore investments with "no tax" and "confidentiality." Aside from three members of a Victoria family, the identities of those wealthy clients remain a mystery to the public.

The court record shows that 15 clients decided to come forward to the Canada Revenue Agency [CRA] after they learned of the investigation. The agency still does not know the identities of at least six more clients who used the scheme, as KPMG has refused to hand over the names.

"Although 15 of KPMG LLP's clients whose identities were sought by the minister have self-identified to the Canada Revenue Agency as of last month, KPMG LLP advises that up to six remain unidentified to the minister," Jamal wrote.

At least four additional clients were already known to the CRA before it made the court application to obtain the list of names and related documents. That brings the list of clients involved in the alleged "sham" to at least 25.

Dennis Howlett, executive director of Canadians for Tax Fairness, said it's about time the case is going to court.

"Had we not been in the middle of an election I fear they would have settled out of court and covered it up and we wouldn't be the wiser."

Still, Howlett said that if 15 multi-millionaire KPMG clients were allowed to "self-identify" it may mean they used the Voluntary Disclosures Program with the Canada Revenue Agency. If so, those clients would not face any penalties, fines, or criminal investigations. They would need to pay back the taxes owed, plus interest, and their files would be closed.

"That's a concern," said Howlett. "It's too easy for big-time tax cheats to get off very lightly ... if you get caught, all you have to pay is the tax you should have paid in the first place."

The government should now turn its attention to KPMG itself, and not just its clients, Howlett said.

"At this point, the court case is only really about KPMG releasing the names of their clients but the real point of it should be to get evidence needed to take KPMG to court for facilitating aggressive tax planning," he said.

CBC News has also learned that KPMG has settled out of court with at least one of its clients who participated in the Isle of Man tax dodge. The terms of that settlement remain confidential.

The NDP has called on the federal Ethics Commissioner to investigate, after CBC News revealed that Finance Minister Joe Oliver, Revenue Minister Kerry-Lynne Findlay and Prime Minister Stephen Harper met publicly with KPMG executives while the CRA probe was alleging the firm tax "sham" had "intended to deceive" the government.