Pret a Manger was not conducting its own checks on manufacturers supplying it with dairy-free products, an inquest has heard.
A coroner is currently hearing details surrounding the death of Celia Marsh, who suffered a fatal allergic reaction on December 27 2017, shortly after eating a super-veg rainbow flatbread.
Mrs Marsh, 42, a dental nurse from Melksham, Wiltshire, had a severe dairy allergy and collapsed in the street after eating the sandwich bought from the chain's store in Bath.
The mother-of-five, who had been on a post-Christmas shopping trip, ate a wrap which was supposed to be vegan but was later found to contain traces of dairy protein.
The yoghurt was produced by Planet Coconut, a UK manufacturer developed by Australian-based yoghurt company CoYo.
Kirsty Langford, a trading standards officer for Bath and North East Somerset Council, told the inquest Pret a Manger had not apparently conducted its own audit of the claims made by Planet Coconut.
She explained that when companies claim a product is dairy-free you would expect retailers to request evidence this can be verified.
She told the hearing: "When a dairy-free claim is made on a product you expect some sort of testing to be taking place.
"That may not itself be the responsibility of Pret A Manger but it would probably be Pret's responsibility to ensure their supplier was undertaking some sort of testing.”
She added that Planet Coconut had said it was testing its products for allergens every year and there was currently no law dictating how frequently this should be done.
An investigation into Planet Coconut found the yoghurt contained few ingredients - primarily coconut cream and 'HG1' starch supplied by sugar giant Tate & Lyle.
The starch was identified as the possible source of the contamination.
Ms Langford pointed out that Tate & Lyle had never claimed the HG1 starch was suitable for dairy-free and a bag of starch carried the warning: "Manufactured in a factory that handlesmilk, eggs, cereals containing gluten, sulphur dioxide and sulphites."
She said Tate & Lyle had said the information concerning the risk of contamination in its products was passed to Planet Coconut, though Planet Coconut denied this.
The court heard evidence that the trace of milk in the flatbread was so small, that it would only be expected to have an effect on around 5% of people with the most acute dairy allergy.
Mrs Marsh's husband Andy said his wife had "religiously" avoided dairy after suffering a near-fatal incident a few months previously in which she needed 15 shots of adrenalin.
Pret was charged with food safety failures in the wake of Mrs Marsh's death, but the prosecution was later dropped due to lack of evidence.
The inquest, which is taking place at Ashton Court Mansion House in Bristol, continues and is expected to last between two and three weeks.