Five of the biggest high street restaurant and coffee shop chains gave out false or misleading information on potentially deadly allergens to customers, according to a report.

Undercover journalists working for the BBC visited a host of sites, where they posed as customers with allergies. They said they were given incorrect advice at branches of Pizza Hut, Nando’s and Frankie and Benny’s, as well as at the coffee shops Starbucks and Costa.

The investigation came after a coroner said he would write to the government over the death of Natasha Ednan-Laperouse, who suffered an anaphylactic reaction after eating a sandwich from Pret a Manger that did not have any allergen advice on its wrapper.

The latest revelations, to be aired in an edition of the BBC’s Watchdog Live programme, prompted criticism from Rasel Shahid, whose sister Shahida died in 2015 after being served a chicken burger that had been marinated in buttermilk, despite telling restaurant staff she had a dairy allergy.

“Shahida’s death was entirely preventable … If buttermilk was labelled properly and correctly on the menu, Shahida would still be here today, so that’s how effective it would have been just to have correct labelling,” he told the programme.

The undercover reporters visited five branches of six restaurant chains and found that only Pizza Express gave clear, correct allergen information on every occasion.

Frankie & Benny’s

At one branch, a journalist was incorrectly told a dish did not contain celery. At another, a reporter was asked to agree to terms and conditions saying the restaurant could never guarantee a dish was completely free of any allergen, barring gluten. The staff member told the BBC’s reporter the form “saves our back”.

Its parent company said it was “deeply concerned” by the BBC’s report. “We fully understand the need for detailed food information and take our obligations on allergens extremely seriously.”

Costa Coffee

At one branch, the BBC’s reporter was incorrectly told a mince pie did not contain milk – despite a staff member consulting the store’s allergy book.

The firm said: “On this occasion, the team member failed to follow correct procedure and provided the wrong information. This is clearly unacceptable and we have reissued guidance and best practice to all stores.”

Pizza Hut

The allergy book was incomprehensible to the reporter and a staff member as they tried to ascertain whether or not two dishes contained mustard. Each was listed as containing the ingredient online.

A Pizza Hut Restaurants spokesperson said: “In this specific instance, the information in the book provided was correct. But we have taken the feedback on board and added QR codes to our menu cards this week, which link through to all of our nutritional information.”

Nando’s

A reporter was wrongly told a burger contained no mustard, before the information was corrected.

The firm said its managers usually dealt personally with customers with allergies. “We’re sorry that, on this occasion, this process wasn’t followed perfectly. As a matter of priority, we have since reminded all our employees of the processes in place and their importance.”

Starbucks

A staff member initially said a lemon loaf cake, which has almonds in the ingredients, did not contain nuts – before advising there was still a risk of nut contamination.

Starbucks said safety was its highest priority. “In this instance, we fell short on this commitment and did not meet our own high standards. We have addressed this issue with our team at the store in question and we have been in touch with all of our UK stores to reinforce our standards and expectations.”