The BBC Watchdog Live has found that leading restaurant and coffee shop chains are giving out incorrect allergy advice.

Acting as customers with food allergies, Watchdog Live's journalists covertly filmed staff at Frankie & Benny's, Pizza Hut, Nandos, Pizza Express, Starbucks and Costa. The journalists asked staff if specific dishes contained one of the 14 major allergens, including nuts, mustard and celery – which is often used as flavouring in stocks and sauces.

None of the outlets visited had allergens listed on menus or labels, so customers must rely on staff being able to give accurate information.

Five out of the 30 outlets visited gave the reporters incorrect information.

By law, cafes, restaurants and takeaways should be able to give customers clear information about which dishes contain allergens. 

Out of the six chains, only Pizza Express gave accurate advice in each of the five branches visited.

Starbucks, Costa, Frankie & Benny's and Nando's said that the incidents Watchdog's undercover team experienced fell short of their usual standards. They told the programme they've addressed the issues with staff at both a regional and national level. 

Pizza Hut told the BBC that while the information provided to Watchdog's undercover journalist was ultimately correct, it is now taking steps to make allergy information clearer.  The pizza chain is introducing QR codes on menu cards this week, and will be making the font size of text in its nutritional guide bigger in the new year. 

At a Costa Coffee branch, the reporter asked for a mince pie they knew contained milk, however even after thoroughly consulting the allergy book they were told by staff that it contained soya milk. 

In one Frankie & Benny's restaurant the undercover journalist told staff she had a celery allergy and enquired about eggs royale, which contains celery, according to the company's website. The server did not at any point consult a product information guide or check with the kitchen, but assured the journalist that the dish didn't contain any celery. 

In a branch of Pizza Hut, a reporter asked if the mac n' cheese contained mustard, which is listed as an ingredient on the company's website. A member of staff showed the reporter a book containing allergy advice, but neither the reporter or the staff member could understand the information in the book.

When the reporter asked if the pepperoni pizza contained mustard, he was told that the member of staff couldn't give him any more information than that listed in the allergy book, which was unclear.

At a Nando's restaurant, the reporter asked if a burger contained mustard. The server guessed that it didn't, but on checking the allergen book, discovered that the burger did in fact contain the ingredient.

While in one branch of Starbucks the advice was ambiguous, with a staff member initially telling the reporter the item he'd chosen – a lemon loaf cake with almonds in the ingredients – did not contain nuts, but ultimately advising there was still a risk of nut contamination.

At one Frankie and Benny's restaurant, a reporter was asked to agree to terms and conditions that state Frankie and Benny's can never guarantee that a dish is completely free of any allergen – except for gluten. The server told the reporter the form "saves our back".

Frankie & Benny's owner The Restaurant Group told Watchdog Live that the allergy advice presented to customers to read and tick is not a disclaimer. The firm added that it does not ask its customers to waive their rights.

The findings come three weeks after Watchdog Live found supermarket bakery counters giving incorrect allergy information, prompting Sainsbury's and Asda to pledge to introduce allergy labels on all in-store bakery products.