Louis Roy of the Confederation of National Trade Unions has announced his resignation over incidents at a training session.
He says he wasn't involved, but was present, when members forced open a liquor cabinet after closing time at the hotel where the event was being held.
Roy also says he accompanied a drunk woman to help her get back to her room — and that people were incorrectly casting aspersions on his behaviour.
The union, better known by its French acronym CSN, has 300,000 members in thousands of workplaces and is the second-biggest labour federation in the province.
The 59-year-old Roy was involved in recent negotiations between the now-defeated Charest government and student protest leaders in a failed attempt to settle their differences over tuition hikes.