The sandwich restaurant chain Subway has started to remove a chemical called azodicarbonamide from its bread, which is said to be used to increase elasticity in shoe rubber and yoga mats.

The chemical is also a food additive, used to improve and bleach flour.

"We are already in the process of removing azodicarbonamide as part of our bread improvement efforts, even though it is an approved ingredient by federal regulatory agencies," Subway Canada said in a statement to QMI Agency.

"The complete conversion to have this ingredient out of the bread will be done soon.”

Subway is also removing the chemical south of the border.

Customers have written on Subway's Facebook wall they will boycott the restaurant until the chemical is completely removed.

"Had I known I was eating a substance also found in shoe rubber, I would never have eaten at Subway," one person wrote.

People on Twitter also commented on the use of the chemical.

"Now I know why the smell of their bread has always repulsed me," San Francisco user @tijuanera wrote.

"Subway is removing shoe rubber chemical from their bread. It's unknown as to whether or not they'll be able to bounce back," Los Angeles user @TheRossEverett wrote. "Those of you not loving my joke, I remind you: I'm rubber and you're glue and whatever you say bounces off of me because I'm in Subway's bread."

A petition on the website FoodBabe.com says azodicarbonamide is used in North American restaurants, but not in Europe or Australia.

The people behind the website said it had to speak out after U.S. First Lady Michelle Obama touted Subway's sandwiches as a healthy choice in her campaign against childhood obesity.