Danish food authorities said boiled and peeled eggs, imported from a Belgian supplier, had been found to contain traces of the fipronil insecticide that has seen millions of eggs pulled from supermarket shelves across Europe.

The eggs were mostly sold to cafes and caterers, Danish authorities said, stressing that the level of the insecticide in the eggs was too low to pose a health risk to humans.

The scandal also spread eastwards on Thursday as a tonne of contaminated egg yolk was found in Romania, and 21 boxes of the tainted eggs were discovered in Slovakia.

It also reached Luxembourg, while Britain said it had imported 700,000 eggs from Dutch farms linked to the scandal — far more than first thought. Sweden and Switzerland have also found contaminated eggs.

Fipronil is commonly used to get rid of fleas, lice and ticks from animals but is banned by the European Union from use in the food industry. When eaten in large quantities it can harm people's kidneys, liver and thyroid glands.

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With questions growing about how the contamination happened and whether consumers have been kept in the dark, pressure has grown on the two countries at the centre of the scandal — the Netherlands and Belgium.

In joint raids with Belgium, Dutch prosecutors said they had arrested “two managers at the company that allegedly used the substance at poultry farms,” with Dutch media naming the suspects' firm as Chickfriend.

Farmers in the Netherlands — one of Europe's biggest egg exporters – and Belgium have previously identified Chickfriend as the company that they hired to treat their chickens to eradicate the parasite red lice.

Dutch authorities said the raids also focused on a Belgian supplier of fipronil, named in the media as Poultry-Vision, and another unidentified Dutch company that allegedly colluded with it.

“They are suspected of putting public health in danger by supplying and using fipronil in pens containing egg-laying chickens,” prosecutors' spokeswoman Marieke van der Molen said.

Belgian investigators meanwhile identified 26 people or companies as suspects during the 11 raids by police and food safety agency officers, for offences including fraud and breaking EU food laws.

Investigators seized paperwork, cars, banking details and fixed assets in both countries. They said they had also seized 6,000 litres of “prohibited products” in Belgium.

The joint offensive came despite Belgium earlier accusing the Netherlands of knowing about the problem of fipronil in eggs since November 2016, but failing to inform them until July.

The Netherlands denied the charge.

In Britain, four major supermarket chains have withdrawn some products containing eggs over the scandal, including sandwiches and salads, the Food Standards Agency said.

Luxembourg said eggs sold in branches of the discount supermarket Aldi had been withdrawn, with one batch containing so much fipronil it was unsafe to be eaten by young children.

The eggs at the centre of the scandal have mainly come from the Netherlands, followed by Belgium and Germany. Scores of farms have been shut.

As the probe continues into how the contamination happened, a lawyer for Poultry-Vision said the firm sold it to Chickfriend, but it has not said where it got the substance.