MONTREAL—The Charbonneau commission has a warning for witnesses: lying is not an option.

Francois Theriault, a City of Montreal inspector who appeared before Quebec’s anti-corruption inquiry was arrested on Thursday and charged with committing perjury. The 54-year-old could face jail time if convicted.

Theriault’s name was first brought up at the commission by self-confessed corrupt contractor Lino Zambito last October.

“Francois Theriault?” the confident former construction boss said. “It was really easy to organize contracts with him.”

Zambito testified that Theriault took bribes in exchange for allowing cost over-runs on construction work. The next day, Theriault was suspended without pay by city hall. The inspector took the stand on Nov. 15 and denied receiving bribes—other than accepting hockey tickets and the occasional free meal and bottles of wine at Christmas.

However, police believe Theriault received a $30,000 discount on his Laval property from developer Paolo Catania of Catcan Construction. At the time, Theriault was supervising a costly project on Queen Mary Rd. in 2006.

Quebec's anti-corruption unit says Theriault failed to disclose that key fact at the Charbonneau commission, an omission that led to his arrest on charges of perjury and obstruction of justice Friday morning.

While witnesses before the commission are immune from prosecution, if they lie or omit certain things that protection drops.

“Perjury charges are laid when somebody is lying, under oath, with the intension of misleading the court knowing that what they are saying is false,” the charge is rare in Canada, but criminal lawyer Philip Schneider says the prosecution's goal is obvious: the commission is sending a message.

“Obviously I think they're trying to send a message with these charges that if you're going to testify at this commission, you'll be telling the truth, you should give the complete version and not try to skate on thin ice on various subject matter,” said Shneider.

If Theriault is found guilty of perjury, a jail term is almost automatic.