Pret has launched a pilot scheme that will see ingredients displayed on products at a shop in London's Victoria, but its CEO has warned that extending the scheme nationwide will take time.
Writing on his blog Clive Schlee said: "I know that sticking labels on sandwich boxes may seem simple, and you may be wondering why we don't just get on with it. The reason is that we have to make sure the process is safe, practical and effective.
"The wrong label on the wrong sandwich can be worse than no label at all. Our teams make our sandwiches and salads in small batches (typically around 12 to 15 at a time) throughout the day in over 350 different Pret kitchens all over the country. We have to perfect the process to minimise the chance of human error."
Schlee said that the original labels were a "bit clunky and the pace is slow" but new designs could follow.
The new measures follow the death of 15-year-old Natasha Ednan-Laperouse, who suffered an allergic reaction to sesame baked into one of Pret's baguettes. An inquest into her death saw a coroner brand the chain's allergen labelling "inadequate", with environment secretary Michael Gove suggesting new policy could follow.
The CEO had told the Real Bread Campaign that Pret will be reaching out to other companies in a bid to develop a common approach to allergen warnings, and within his blog he added that lessons learnt from the trial would be shared with both the industry and the government.
Pret is also making moves to comply with a ruling by the Advertising Standards Agency that it should remove the word "natural" from its marketing. The claim has been removed from Pret's website, new store signage and packaging designs and is in the process of being removed from existing stores, with an expectation this will be completed in 2019.