LONDON -- A second customer of restaurant chain Pret a Manger died after eating a sandwich containing an allergen that was not noted on the label, the company has confirmed. The coffee-and-sandwich business has promised to improve its labeling following criticism at an inquest last month into the death of 15-year-old Natasha Ednan-Laperouse, who died in 2016 after eating a Pret baguette that contained traces of sesame.

The company said an investigation was underway into a second case, in which a customer died in December after eating a supposedly dairy-free product that contained dairy protein. BBC News reports that Celia Marsh, 42, from Melksham, Wiltshire, died after eating a "super-veg rainbow flatbread."

Pret a Manger blamed a supplier of its dairy-free yoghurt. But the supplier, CoYo, said Sunday the allegation was "unfounded."

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The parents of Ednan-Laperouse, who are campaigning for stronger allergen warnings, said they were "incredibly saddened to learn of someone else losing their life from allergens in their food."

Nadim and Tanya Ednan-Laperouse, with their son Alex, prepare to speak to the media outside West London Coroners Court, Friday Sept. 28, 2018, following the inquest into the death of Natasha Ednan-Laperouse, 15, seen on poster, who died after suffering a fatal allergic reaction on a flight from London to Nice after eating a Pret A Manger sandwich at Heathrow Airport. Jonathan Brady / AP

Ednan-Laperouse collapsed during a flight from Heathrow to Nice in July 2016, BBC News reported. Her father administered two EpiPen injections, but the teenager died within hours.