A butcher shop in Montreal's Plateau-Mont-Royal borough has been adding horsemeat to hamburger patties advertised as being entirely made of beef.

An investigation by Radio-Canada has found burger patties advertised as being 100 per cent beef from La Maison du Rôti on Mont-Royal Avenue were mixed with both horsemeat and pork.

One former employee, who asked to remain anonymous, told Radio-Canada that it was common practice.

"I often helped chop up meat to make burgers and to also make sausages," the employee said. "They asked me each time to add horse and sometimes pork."

'I will find a solution'

DNA testing carried out at Trent University found that the beef patties, marketed as entirely made of beef by Maison du Rôti, contained between 37 and 46 per cent horse meat. Radio-Canada tested beef patties bought on May 9 and May 16, 2016.

Beef accounted for about 38 to 53 per cent of the patty, and pork made up anywhere between seven and 18 per cent.

The investigation also found that merguez sausages from the butcher, advertised as being made with beef and lamb, also had horsemeat and pork mixed in.

Merguez is a type of North African sausage, usually made of lamb and beef. These sausages were found to also contain pork and horsemeat. (Radio-Canada) Owner Michel Legrand said he wasn't aware that employees were adding horse meat and pork to the company's beef patties. He added that employees didn't want to mislead their customers.

"I will find a solution," Legrand told Radio-Canada. "I will make sure to fix this mistake immediately."

This kind of meat combination and false advertising is more common than people think, according to Sarah Berger-Richardson, who studies food law and policy at McGill University.

"A lot of people are buying food, meat and fish that they think is one thing and it's not," Berger-Richardson said.

The ingredients of the beef patties are advertised as beef, onions and spices. (Radio-Canada)

In Montreal, businesses that mislead customers regarding the composition of food are subject to fines ranging from $500 to $9,000.

The Maison du Rôti employs close to 95 people and supplies meat to over 400 hotels, institutions and restaurants, according to its website.

It says it has changed its ways since being confronted with the results of the Radio-Canada investigation.