LONDON — On the way to a vacation in France with her father, Natasha Ednan-Laperouse picked up a sandwich at a Pret A Manger store in Heathrow Airport. She read the label carefully, because she had a severe sesame allergy.

But the artichoke, olives and tapenade baguette she chose had sesame seeds baked into its dough, and carried no mention of the fact. Ms. Ednan-Laperouse, 15, collapsed from a cardiac arrest during the flight, and was dead within two hours.

On Friday, an inquest at the West London Coroner’s Court found the labeling of the sandwich had been “inadequate,” because it did not warn of the “hidden” sesame seeds.

“There was no specific allergen information on the baguette packaging or on the food display cabinet, and Natasha was reassured by that,” the coroner, Sean Cummings, said in his ruling.