In large quantities, the insecticide is considered "moderately hazardous" by the World Health Organization, and can have unsafe effects on people's kidneys, liver and thyroid glands.
Belgium has been aware of a potential problem with fipronil in the poultry sector since June, its food safety agency admitted, adding that it did not say anything because of a fraud investigation.
Eggs contaminated with the "potentially harmful" pesticide fipronil have been distributed in the United Kingdom, after warnings from the European Union sparked panic across the continent.
Belgium meanwhile vowed full transparency about why it kept the scandal secret despite originally learning in June about the problem involving fipronil, a substance potentially risky to humans.
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The contaminated egg scandal in the Netherlands has led to empty shelves in the supermarkets and it will be at least until the end of the week before supplies are back to normal, according to food retail umbrella group CBL.
Photo Farm workers discarded eggs in Onstwedde, the Netherlands, last week after concern about insecticide contamination.
The watchdog initially stated on Friday (4 August) there was "no evidence that UK-produced eggs are affected or that contaminated products have entered the United Kingdom market".
"The number of eggs involved represents about 0.0001% of the eggs imported into the United Kingdom each year", it said.
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The agency is "urgently investigating" the issue, but to the best of their knowledge, the affected products are no longer on shelves.
It is believed the toxic substance was introduced to poultry farms by a Dutch business named Chickfriend brought in to treat red lice, a nasty parasite in chickens.
Meanwhile Dutch farming organisation LTO said several million hens may need to be culled at 150 companies in the country, with 300,000 having already been killed.
About 180 poultry farms in the Netherlands have also been temporarily shut in recent days while investigations are held.
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The European Union on Monday notified the food safety authorities in Britain, France, Sweden and Switzerland to be on the lookout for contamination in eggs after a food scare in Belgium, Germany and the Netherlands.