Pret a Manger failed to conduct checks on manufacturers supplying dairy-free products, an inquest has heard.
Celia Marsh suffered a fatal allergic reaction in December 2017 after eating a super-veg rainbow flatbread.
The 42-year-old, from Wiltshire, had a severe dairy allergy and collapsed after eating the sandwich bought from the chain's store in Bath, Somerset.
A trading standards officer said they did not have any evidence of Pret verifying the dairy-free claim.
The wrap had contained yoghurt which was supposed to be vegan but was later found to contain traces of dairy protein.
The yoghurt was produced by Planet Coconut, which is the UK manufacturer and distributor of products developed by Australia-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.
"When a dairy-free claim is made on a product you expect some sort of testing to be taking place," she said.
"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."
Ms Langford continued: "When we went back to Planet Coconut we weren't supplied with any test certificate and we weren't sure that Pret had been supplied with any either - we haven't been supplied with any evidence of that.
"That becomes more important when there is a dairy-free claim on the product, that is when you would expect finished product testing to make sure that 'free-from' claim can be verified."
Ms Langford said that ordinarily, a free-from claim on a product would be supported by a risk assessment by the retailer checking all the processes and ingredients in the supply chain.
She recounted that Planet Coconut had said it was testing its product for allergens every year and added how often a product should be tested was not enshrined in law.
"With a small supplier like Planet Coconut it would be less often than a bigger (company)," Ms Langford said.
"I would expect Pret to check that testing was being undertaken through their supplier - there were supplier audits being undertaken (by Pret) but I don't think the audit picked up on the testing of the product."
An investigation revealed the yoghurt contained 'HG1' starch, supplied by sugar giant Tate and Lyle, that was identified as the possible source of the contamination.
The court was told Tate and Lyle had never said the HG1 starch was suitable for a dairy-free claim and alleges it had passed on the information concerning the risk of contamination to Planet Coconut.