Pret a Manger has been hit by further controversy over the weekend, as it emerged that a second customer had died after eating one of its sandwiches in 2017.

The sandwich chain found itself in a escalating row with a former supplier, after it blamed the second death on an ingredient supplied by the vegan brand CoYo, a claim the yoghurt company said was unfounded.

It emerged on Saturday that a Pret customer had died in 2017 after eating a “super-veg rainbow flatbread” that was supposed to be dairy-free.

Pret said it had been mis-sold yoghurt that forms one of the ingredients of the flatbread and which was guaranteed dairy-free but was found to contain dairy protein.

The company agreed to full labelling of ingredients on all its freshly made products last week following the case of 15-year-old Natasha Ednan-Laperouse, who was allergic to sesame and died on a flight after eating one of its baguettes bought at Heathrow airport.

Following the news of another death, Natasha’s parents, Nadim and Tanya Ednan-Laperouse, said: “We were incredibly saddened to learn of someone else losing their life from allergens in their food. Our hearts go out to the bereaved family.”

The pair have been campaigning for allergy awareness and a change in food-labelling laws.

The second customer, who has not been named, collapsed and died on 27 December after buying the sandwich in a shop in Stall Street, Bath.

Bath’s council alerted Pret to the incident and the chain said it withdrew all affected products. The firm said it had ended its contract with CoYo and was taking legal action.

“Subsequent testing by Pret and two independent authorities found that the CoYo dairy-free yoghurt contained traces of dairy protein,” a Pret spokesman said. “This is believed to have resulted in the tragic death of a customer from an allergic reaction in December 2017.

“Our deepest sympathies are with the family and friends of our customer in this terrible case and we will seek to assist them in any way we can.”

CoYo, a coconut milk brand that the TV cook Nigella Lawson has endorsed, said on Sunday that Pret’s claims that it was to blame were unfounded. It accused the sandwich chain of hampering its own investigation into the death by failing to provide vital information.

“Pret’s inability to provide us with a batch code, despite several requests, has severely limited our ability to investigate this further,” it said

CoYo recalled its yoghurts in February 2018 after dairy traces were found. The Food Standards Agency investigated together with Bexley council in London, where CoYo is based, before it issued an allergy alert.

CoYo denied on Sunday that the product recall was related to the death. “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,” a spokeswoman said.

The company would continue to help to find the true cause of the death, she said. “We urge all parties to work together, and not to speculate on the cause of this tragic death which is unknown as far as we are aware and is still being investigated by the coroner’s court.”

Following the February recall, Coyo said it had been devastated to discover dairy in its yoghurts, “especially as being dairy-free sits at the heart of everything we do”.

It said it had carried out a “thorough review” of its ingredients and identified that the contaminated material was provided by an external third-party supplier, whose relationship had now been terminated.