EGG inspectors have been on alert for rogue traders passing off factory farmed eggs as free range for many months after an anonymous tip-off.

The scam has been an industry secret for years, but the problem for honest producers and wholesalers is that is is almost impossible to distinguish between a free-range and standard egg with the naked eye.

With shop prices for free-range double those for standard boxes — about 90p to £1 for six free-range, compared with 40p to 50p for standard — there are rich pickings to be made by passing off battery-farmed eggs.

Consumer demand for free-range is rising fast and the business is now worth some £220 million a year for 1.75 billion free-range eggs sold in supermarkets and small shops. This premium product is 35 per cent of the retail market.

A team of 30 officers is employed by the Department for Environment, Food and Rural Affairs to police the EU egg marketing regulations.

Defra inspectors are able to follow an audit trail. Every egg on sale in the EU must have a producer-distinguishing number. On all Class A eggs, this comprises the figure 1, which denotes free-range production, “UK” to show country of origin, then a five-digit producer number Paperwork is vigilantly scrutinised to ensure that production records tally with supplies. These numbers can then be verified further up the supply chain, with records held by egg wholesalers, packing houses, and the egg boxes on sale in supermarkets.

Gaps in paperwork prompt further investigations and the eggs themselves are then scrutinised.

Inspectors make random visits to premises, although weekly visits are expected at the country’s biggest egg suppliers, such as Dean’s and Stonegate — companies that are hoping to merge to form Noble Foods, though this subject to a Competition Commission investigation. Individual eggs are checked using ultraviolet light, which can pick out certain marks on the shell.

If certain lines appear on the egg surface at 2.5cm intervals, this can indicate that the chickens laid their eggs on wire surfaces, which are used in battery cages.

Similarly, free-range eggs can show other tell-tale signs, such as use of rest boxes or contact with Astroturf surfaces.

A Defra inspector said: “These lines cannot give us the proof beyond reasonable doubt, but when matched with paperwork we can identify unscrupulous operators.”

It is understood that, in the scam, imported eggs were allegedly stamped with bona fide egg producer numbers. A total of 1.4 billion eggs are imported annually.

Weight of eggs is also checked by the inspectors to ensure that small eggs are not being passed off as medium or large, which are the bulk of all sales and command higher prices. A small egg weighs 53g or less, a medium 53g to 63g, a large 63g to 73g and an extra large 73g.

To date Defra has uncovered little fraudulent activity and the highest number of cases in any one year is six — and these are usually connected with that quality of egg than passing off factory-farmed eggs as free-range.

The current police investigation has shocked the country’s 300 free-range producers and other egg companies. They are all demanding speedy action from enforcement authorities to clarify the situation.

One said: “Without some action a slur hangs over our business. If it is going on it must be dealt with because it is cheating the consumer and the farmers.”

In a statement, the RSPCA said that it was appalled by news of a possible scam.

It said that the free-range sector had grown massively from 21 per cent of the the total market in 1998 to at least 35 per cent today and was a major step forward in the welfare of laying hens.

It urged consumers to look out for its Freedom Food label on eggs.

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The average person eats 172 eggs every year

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