“Fresh, slivered onions served on Quarter Pounders and other menu items from McDonald’s are the likely source of this outbreak,” the Centers for Disease Control and Prevention said.

These particular onions were supplied by Taylor Farms, which issued a voluntary recall. McDonald’s also pulled the onions.

Restaurants in a dozen US states temporarily pulled the burgers from their menus this month amid the outbreak linked to them.

The number of people sickened now stands at 90, including 27 who had to be hospitalised, and an elderly man in Colorado died.

McDonald’s on Sunday said it would resume selling its Quarter Pounder hamburgers after tests for E coli came back negative.

The E coli bacterium causes stomach cramps, diarrhea and vomiting that last three to four days. Most people recover without needing treatment but some develop complications.