Traders may be incorrectly labelling more than one million imported battery eggs each week and then selling them as free range, say government officials.
Defra is investigating the scale of egg fraud in Britain
Last year, the Department for the Environment, Food and Rural Affairs began an investigation into the scale of egg fraud in Britain.
It found trading in imported battery eggs from two European countries may have taken place for up to five years.
In that time, up to 10 lorries of eggs came into the UK each week, Defra said.
Demand for free range eggs has soared in recent years, to the extent that British farmers have struggled to keep up.
Last November, three people were arrested over alleged fraud at a West Midlands egg packing firm. They have since been released on bail.
Defra began investigating whether eggs from caged hens were being sold as free range.
It says it is imperative to get to the bottom of any egg mislabelling because it would be a fraud against the British public and the British egg industry.
The difference in price can be between 40 and 50 pence for six eggs.