Two supermarket groups believe that packets of standard cornflakes have been mixed with the honey nut flavoured version. Nut allergies affect 1 per cent of the UK population and cause five deaths a year.
Proteins in the nut trigger an immunological reaction which causes the body to go into anaphylactic shock, in which all the major systems shut down.
The packets involved are Sainsbury's 500g Economy Cornflakes and Somerfield Basics cornflakes with a product number of 7064 and a best-before date of March 1998.
The scare began when Sainsbury's received a call on Wednesday evening from a family which had discovered a "handful" of honey nut cornflakes in two packets of economy cornflakes.
The discovery was made before a relative with a nut allergy ate any of the flakes.
The contamination was traced back by Sainsbury's to a supplier which also produces Basics cornflakes for Somerfield and Gateway stores.
Although tests on dozens of other packets found no further traces of honey nut cornflakes, both supermarket chains have withdrawn the packets from shelves and offered refunds to customers.
A Sainsbury's spokeswoman said: "This is not a general major food safety scare but is of importance to the small number of people with nut allergies."
Between 100,000 and 150,000 packs of Happy Shopper cornflakes produced by the same supplier have also been withdrawn by Nurdin & Peacock.
The 500g packets were taken from the shelves of 40,000 independent retailers.
In 1993, David Reading, of Hampshire, began campaigning to raise awareness of the dangers of nut allergy after his 17-year-old daughter Sarah died after she ate a slice of lemon meringue pie which contained nuts. His Anaphylaxis Campaign gained 2,000 members in its first year.
Symptoms of the allergy include breathing difficulties, rashes, itching and swelling of the face.
Nut allergies are so widespread that two weeks ago British Airways said savoury cocktail biscuits were replacing peanuts as the standard drink accompaniment for economy passengers worldwide.Reuse content