In large quantities, fipronil, is considered to be "moderately hazardous" according to the World Health Organisation, and can have dangerous affects on people's kidneys, liver and thyroid glands.

Asda, Morrisons, Sainsbury's and Waitrose are all urgently withdrawing millions of fresh salads, sandwiches and fillers which contain the infected eggs.

Fresh eggs are largely unaffected, with contaminated eggs instead present in processed foods in which they are one ingredient among many others, mostly used in sandwich fillings or other chilled foods.

The eggs were imported to the UK between March and June, the Food Standards Agency said.