London (CNN)A second person has died from an allergic reaction to one of Pret a Manger's products, the food chain confirmed Sunday.
The customer died in December 2017 after eating a "super-veg rainbow flatbread" that contained a yogurt that was supposed to be dairy-free.
But after testing by Pret and two independent authorities, it was later found that the ingredient, supplied by yogurt producer Coyo, contained dairy protein. The Food Standards Agency (FSA) was informed and all Coyo products were recalled nationwide.
Pret said in a statement that "their deepest sympathies are with the family and friends of our customer in this terrible case and we will look to help them in any way we can." The company said it was taking legal action against Coyo.
Coyo denied its product was the cause of the fatal allergic reaction. In a statement posted to its Twitter feed Sunday, the company said "the claims made by Pret are unfounded."
"The dairy-free product we provided to Pret in December 2017 at the time of this tragedy, is not linked to the product we recalled in February 2018," it concluded. The company said the cause of death was unknown and still being investigated by coroners.
Pret a Manger has recently been under scrutiny following the death of a 15-year-old girl who had an allergic reaction to a sandwich containing sesame seeds.
Natasha Ednan-Laperouse fell ill on a British Airways flight to Nice, France, after buying an olive tapenade and artichoke baguette from a branch of Pret a Manger at London Heathrow Airport. She died later of anaphylactic shock, a UK medical coroner ruled last week.
Pret a Manger Chief Executive Clive Schlee said in a statement that the fast-food chain was "deeply sorry" for her death. He said he hoped the food-labeling measures would "set us on course to drive change in the industry so people with allergies are as protected and informed as possible."
The company will begin a trial of full ingredient labeling from November and in the coming weeks will ensure that "prominent allergen warning stickers are placed on all individual freshly made products" and add additional allergen warning signs in shops, it said in a statement seen by CNN.