Belgium has admitted it knew in June that eggs from Dutch farms might be contaminated with an insecticide - a month before the issue became public.
The information was not shared because of a fraud investigation, a spokeswoman for Belgium's food safety agency said.
Tests found the chemical fipronil, which can harm people's kidneys, liver and thyroid glands, in Dutch eggs.
Supermarkets in Belgium, Germany and the Netherlands have removed potentially contaminated eggs.
"We have known since early June there was potentially a problem with fipronil in the poultry sector," spokeswoman Katrien Stragier told reporters.
"We immediately launched an investigation and we also informed the prosecutor because it was a matter of possible fraud," she added, without giving more details.
German Agriculture Minister Christian Schmidt expressed concern about the revelation, and planned to call his Belgian counterpart on Monday.
One German official said up to 10 million of the contaminated eggs may have been sold in Germany.
The Netherlands is Europe's largest exporter of eggs and egg products, and one of the biggest in the world. It exports an estimated 65% of the 10 billion eggs it produces every year.
About 180 poultry farms in the country have been temporarily shut while investigations are held.