Ontario fire fighters are battling their union over so-called 'double-hatting' – that’s when a fulltime firefighter in one city also serves as a volunteer firefighter in a smaller community.

An internal trial was held Monday by the International Association of Fire Fighters (IAFF) for four Mississauga firefighters who were double-hatting and they could end up losing their jobs.

That’s a prospect that has local fire departments worried.

Chief Rick Pedersen of the Woolwich Fire Department says 10 double-hatters work for him, but he is concerned they could leave depending on the outcome of the Mississauga case.

“We would lose [their] experience and their availability. And it's hard to attract competent volunteers to fight fires,” said Pedersen.

The IAFF, the union for fulltime firefighters, has a rule forbidding members from volunteering for other unionized fire departments.

Breaking that rule is what landed the Mississauga firefighters in trouble. Fire departments in Waterloo Region's townships are not unionized.

“Anything that would jeopardize them participating or volunteering for their local community in which they live would be immensely unfortunate,” said Michael Harris, the Progressive Conservative MPP for Kitchener-Conestoga.

The IAFF says a panel of three union members are now deliberating the case of the Mississauga firefighters.