A new conflict-of-interest complaint against Toronto Mayor Rob Ford and his brother Doug Ford has been filed in court.

In the latest dramatic twist in the closely watched mayoral race, Toronto resident and activist Jude MacDonald filed the complaint with the Ontario Superior Court, alleging that both Rob and Doug broke the Ontario Municipal Conflict of Interest Act by participating in council debates and votes on matters related to clients of the family firm, Deco Labels and Tags.

The case is not likely to wrap up before this month's election, but it means possible court proceedings even if both brothers are elected on Oct. 27. Rob, who dropped out of the mayoral race after he was diagnosed with cancer last month, is running for council in Ward 2. Doug is running for mayor in his place.

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If successful, Ms. MacDonald's complaint, based in part on reports published in The Globe and Mail, could result in both brothers being removed from office and disqualified from running again for up to seven years.

"I think elected representatives have a lot of power. We're talking about a $9-billion budget. I think it is reasonable to ask elected politicians to abide by a law that is there … for the common good," said Ms. MacDonald, a civic activist who has frequently criticized Mayor Ford.

She said she was not acting as a part of an organized political body, and decided to pursue the case because "to me, democracy is not a spectator sport. Someone's got to step up."

Doug Ford described the case on Friday as a "political witch hunt," and questioned Ms. MacDonald's motivations.

"I've got to question how someone gets away with this nonsense, spending the court's money, the taxpayers' money, to go on a political witch hunt."

When asked if the allegations are true, he said: "Well, no. We went through this already, guys. We went through this a few months ago. If I wasn't running for mayor, she wouldn't be doing it."

Ms. MacDonald's complaint alleges the Ford brothers debated or voted on issues at city council and committee related to companies that were reported by The Globe to be Deco clients, including Nestlé Canada Inc., Coca-Cola, Cara Operations, Porter Airlines and Soheil Mosun Ltd.

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The Municipal Conflict of Interest Act prohibits council members from participating in matters in which they have a pecuniary interest. It also requires council members to disclose such an interest.

Tim Gleason, a lawyer for Ms. MacDonald, said they decided to pursue the allegations in court rather than filing a complaint with the office of the city's integrity commissioner because the legal process offers stronger remedies – including the power to declare a member's seat vacant, or disqualify him or her from office for up to seven years.

Mr. Gleason added that his client would like to see reform to the Municipal Conflict of Interest Act, which requires private citizens to bring forward a complaint in order for a court case to be triggered.

"It really puts an onerous demand on individuals that I don't think is realistic or effective," he said.

Now that Ms. MacDonald's application has been filed with the court, the Ford brothers have 20 days to respond.

In response to the allegations, Rob has said in the past that "no one, but no one, can buy the Fords."

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This is not the first time the mayor has faced a conflict-of-interest case in court. In 2012, his seat was briefly declared vacant after resident Paul Magder filed a complaint against him for participating in a vote to let himself off the hook for failing to repay $3,150 in improper donations to his football foundation. The Ontario Divisional Court later overturned that ruling.