CBC News has learned Independent Senator Mac Harb is launching a legal challenge in Ontario Superior Court against the Senate.
Court documents are expected to be filed Friday or Monday at the latest.
Harb's lawyers are expected to argue he was not given due process or a fair hearing by the Senate.
Harb's lawsuit is expected to claim the process that found he had inappropriately claimed more than $230,000 in living expenses was tainted and flawed.
Earlier this week, Harb was ordered to pay back $51,000 within 30 days. He was also given the option to repay a total of $231,649 if he does not want to face an extended and detailed audit of his expenses dating back to at least 2005.
The information on the order to repay comes from a copy of a letter from the Senate internal economy committee dated June 3. The $51,000 was included in a Senate report released on May 9, after an outside audit conducted by the accounting firm Deloitte gave its findings about how much time Harb spent in the house he called his primary residence near Pembroke, Ont., during an 18-month period.
Since then, Senate staff has looked at the amount Harb charged for maintaining a secondary residence in Ottawa dating back to 2005, and has calculated he owes a total of $231,649 for mileage and living expenses, including interest. The total includes the $51,000.
At a May 9 meeting, the 15-member Senate committee that monitors senators' expenses reported it does not believe Harb's Pembroke, Ont. home is actually his primary residence.
Senators are allowed to charge living and travel expenses if their primary residence is at least 100 kilometres from Ottawa.
Harb resigned from the Liberal caucus in early May to sit in the Senate as an Independent until issues with his expenses are resolved.