Pret a Manger did not label artisan baguettes as containing sesame, despite six allergic reaction cases in the year before a teenager died after eating one.
Natasha Ednan-Laperouse suffered a fatal reaction to an artichoke, olive and tapenade sandwich bought from a branch of the outlet on 17 July 2016, unaware the it contained the plant which she was extremely allergic to.
An inquest into the 15-year-old's death heard that the product's packaging failed to mention that it was contained in the dough.
Food companies are required by European Union (EU) regulations to warn customers about allergy risks. They can do this either on signs and packaging or orally, usually meaning they are told to enquire themselves.
Pret a Manger chooses to deliver the information orally and is supposed to have stickers within fridges telling customers to ask staff members for details.
The company's complaint log from between 17 July 2015 and 29 June 2016 records nine cases of sesame-related allergy incidents.
Six of the nine cases involved baguettes that were described as "posh" or "artisan".
One woman involved in one such incident contacted the law firm representing Natasha's family, after learning about the teenager's death.
She told them that she nearly died after suffering an anaphylactic reaction to the sesame in a baguette "nine months before Natasha's death", Jeremy Hyam QC told the inquest at West London Coroner's Court.
Her father, a doctor, was present and saved her life, she said, adding that she was 17 at the time.
The woman's mother later contacted Pret a Manger, as she was shocked to learn that allergy information was only available upon request.
"My mother expressed her alarm at this and warned that, in her opinion, other similar adverse events could easily occur," the email to Leigh Day, the law firm, said.
My Hyam said that the complaint was a "specific warning" but that the company still failed to label their sandwiches with the allergy information.
"There was a clear concern being repeatedly raised that artisan baguettes were causing sesame seed allergy problems, which were not properly responded to by Pret," he said as he questioned Jonathan Perkins, the food chain's director of risk and compliance.
Mr Perkins insisted: "We responded appropriately to each individual complaint at the time."
Pret a Manger is supposed to have signs in its stores which advise customers with allergies to ask staff for details.
But a photo of the branch where Natasha bought her sandwich, taken eight days after her death, appears to show a fridge without an allergy sticker.
Heathrow Airport's Terminal 5 outlet ordered dozens of allergy stickers in 2016, but questions were raised at the inquest about whether they were used.
A food officer from Hillingdon Council reported that there were no allergy stickers to be seen at the store's tills when she visited the premises in 2017.
Mr Perkins was repeatedly asked about whether the lack of stickers amounted to a "serious failing" on the company's part.
"I will accept that word 'serious'," he said.
When asked about what he had learned, Mr Perkins said that "the parent" in him "would want to change everything," but "the professional in me has to remain grounded."
He added: "The learning for me is that humans are fallible in their actions, we will at some times get things wrong. We need to make (allergy policy) as simple as possible, we need to make it as consistent as possible - it cannot be one or the other. There is more that can be done and we are committed to doing more, my view is the improvements that we made are sound and just improvements, but we cannot stop improving."
She went into cardiac arrest and later died in hospital in Nice.
The inquest will continue until Friday.
Press Association contributed to this report
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies