Natasha Ednan-Laperouse, 15, suffered a fatal reaction to a sandwich from the outlet, which she had not realised contained the ingredient, on July 17, 2016.
An inquest at West London Coroner’s Court was told the packaging failed to mention that sesame seeds were “hidden” in the dough.
On Tuesday, it emerged that a “specific warning” about the dangers of not signposting the allergen had been given to the food chain the previous year.
Pret baguettes described as “posh” or “artisan” were said to contain sesame seeds, including the artichoke, olive and tapenade one that Natasha ate.
A complaint log for the company from between July 17 and June 29, 2015 showed nine cases of sesame-related allergy incidents.
Four of these led to customers seeking hospital treatment, while another went to a medical centre.
Six of the nine cases involved “artisan baguettes”, including one woman who contacted the law firm representing Natasha’s family after learning of her fate.
Jeremy Hyam QC told the hearing that the woman had nearly died after suffering an anaphylactic reaction from sesame in the sandwich “nine months before Natasha’s death”.
Her father, a doctor, was present and helped prevent the episode turning fatal. She was 17 at the time.
The woman’s mother subsequently contacted Pret customer services as she was shocked to learn allergy information was only available upon request, the email to Leigh Day said.
“My mother expressed her alarm at this and warned that, in her opinion, other similar adverse events could easily occur,” it said.
Hyam said this was a “specific warning”, but Pret still failed to label sandwiches with allergy information.
Questioning Jonathan Perkins, the chain’s director of risk and compliance, he said: “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.”
Perkins said: “We responded appropriately to each individual complaint at the time.”
More than a year after the complaint, Pret changed the design of a label within its fridges that tells customers to ask staff for allergy information.
Under EU regulations, food companies are required to warn customers about allergy risks either on signs and packaging or orally, usually meaning they are told to inquire themselves.
Pret chooses to deliver allergy information orally and is supposed to have stickers within fridges telling customers to ask staff members for details.
But a photo of the branch where Natasha bought her sandwich, taken eight days after her death, appeared to show a fridge without the allergy sticker.
Heathrow Airport’s Terminal 5 outlet ordered dozens of stickers for the fridges and tills in 2016, the inquest heard on Monday, but questions were raised about whether they were used.
A food officer from Hillingdon Council reported that there were no allergy stickers to be seen at the tills when she visited in 2017.
Perkins was challenged repeatedly that, if accurate, the lack of stickers amounted to a “serious failing”.
“I will accept that word ‘serious’,” he told the hearing.
Hyam suggested Pret’s approach lacked consistency, as it labelled some products “gluten free” and gave ingredient outlines on fridge shelves.
The chain changed the name of its chocolate croissant to reflect the fact it had hazelnut in it, a move which the lawyer said reflected “quite a serious failing”.
Natasha, from Fulham, south-west London, collapsed on a British Airways flight from London to Nice, on her way to a four-day break with her father and best friend.
The teenager suffered from numerous allergies and reacted badly to the sesame seeds, which caused her throat to tighten and vicious red hives to flare up across her midriff, eventually triggering cardiac arrest.
Two epipens were jabbed into her legs, but the symptoms did not abate and she was declared dead the same day at a hospital in Nice.
Perkins told the inquest: “I accept that there are a number of individuals who have had a very negative experience and tragic experience of consuming that, but I also look at all the customers and thousands of allergy sufferers who come through our doors and are able to shop safely.”
The inquest is due to last until Friday.