A justice of the peace has ruled that a "no left turn" sign in Toronto is unenforceable because it is not written in both English and French.

The ruling Monday by justice of the peace Alice Napier could result in thousands of traffic tickets being dismissed.

Lawyer Jennifer Myers argued that a traffic sign in downtown Toronto violated the Highway Traffic Act and the French Language Services Act because it was not in both official languages.

Napier agreed at a night court hearing Monday, and threw out a ticket issued to Myers for making an illegal left turn. Myers does not speak French.

Daniel Brown, a law student who represented her in court, said Myers' victory could prove expensive for the city of Toronto.

"Certainly there will be some cost consequences," Brown said. "They will have to update every traffic sign to correspond with the French Language Services Act."

Mayor David Miller said Tuesday that the city would appeal the decision.

George Bartlett, director of prosecutions for the city of Toronto, said a provision of the Highway Traffic Act that requires French language signs does not apply because city council never approved it.

Bartlett also said that a decision by a justice of the peace would not set a precedent that invalidates other tickets issued on the basis of English-only signs.