Canada is implementing tighter restrictions on romaine lettuce imports, following numerous E. coli outbreaks linked to U.S.-grown product in recent years.

Starting on Wednesday, the Canadian Food Inspection Agency will require that importers prove that their product does not come from California's Salinas Valley or that their lettuce has "below-detectable" levels of E. coli in laboratory testing.

The move comes after several outbreaks of E. coli have been linked to farms in the area, the last in late 2019.

"Food safety investigations by Canadian and U.S. authorities identified the Salinas Valley growing region as a recurring source of the outbreaks," the agency said in a statement posted to its website.

"To mitigate risk in the event of another outbreak this fall, the Canadian Food Inspection Agency is implementing temporary import measures aimed at preventing contaminated food from entering the marketplace."

There are numerous ways leafy vegetables can become contaminated with E. coli, including being grown in improperly composted manure.

The new restrictions will apply to romaine lettuce and mixed salad products that contain romaine lettuce, and will be in force until Dec. 31.