Exposed: How cheap factory-farmed meat, fish and eggs are passed off as luxury goods costing shoppers billions

There is mounting evidence of battery farm eggs being sold as free range, farmed fish passed off as wild and inferior meat labelled as organic.

It is also feared that premium products are being adulterated to boost profits. For example, ordinary virgin oil is dyed dark green with chlorophyll to make it look like extra virgin.

As much as 10 per cent of food sold through high street grocers and restaurants is not what it seems, says the consumer group Which?.

The watchdog also raises the alarm over GM ingredients finding their way into the national diet - without being declared - through rice, soya and cooking oils.

News of the large-scale fraud came as it emerged some U.S. stores are introducing rice rationing to deal with spiralling costs.

Prices in Thailand, the world's top exporter, have trebled since the start of the year to $1,000 a ton.

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High-value products are the most likely to be chosen for food fraud.

For example, wild salmon will cost 40 per cent more than the farmed equivalent, while free-range eggs are around 80 per cent more expensive than those from battery farms.

It is also profitable to substitute cheap alternatives for foodstuffs that are in short supply, such as free-range eggs, organic vegetables, basmati rice, extra virgin olive oil, buffalo mozzarella cheese and arabica coffee beans.

Alcohol, such as vodka, is also being counterfeited and the consequences can be more serious than a financial loss. High levels of methanol in counterfeit versions can cause blindness and death.

The allegations of food fraud stretch from independent takeaways to national supermarkets, school canteens and leading restaurants.

In the past, firms such as Sainsbury's, Tesco and Harrods have been put in the dock by the Government's Food Standards Agency.

Which? claims that a huge number of shoppers are being duped into paying over the odds for suspect food.

"The level of food fraud in the UK has been estimated at about 10 per cent, or £7billion a year, but the true extent is impossible to gauge," a spokesman said.

"Food fraud is not always obvious. It is not easy for consumers to spot when a premium product has been substituted or mixed with a cheaper one or when a label lies about its origin."

She added: "Unscrupulous fraudsters have developed new ways to fool consumers and increase profits.

"Apart from health risks, the most serious effect of food fraud on consumers is financial. Paying £10 for what you believe to be an organic, free-range chicken that is actually a battery-farmed bird that you could have bought for £2 leaves you considerably out of pocket."

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Last year, Tesco was found to be selling expensive 'corn-fed' chicken that were actually raised on a cheaper diet. It apologised for an 'accidental mistake' by a farmer.

Last year also saw one of the country's biggest restaurant chains, Mitchells & Butler, prosecuted for a series of food fraud offences.

The company behind Vintage Inns, O'Neills, All Bar One and Browns made misleading claims to dress up the origins of its ingredients and dishes in some outlets.

In one case a buffalo mozzarella turned out to be a cheaper full-fat cow's cheese. The company blamed an administrative error.

Leading stores, including Sainsbury's and Harrods, were accused of selling cheap farmed fish as wild last year.

An FSA investigation across the high street found that one in ten supposedly wild sea bass and sea bream, which are among the most exclusive and sought-after fish, were farmed.

The proportion rose to almost one in seven of 'wild' salmon samples. The stores involved all insisted that their fish came from accredited suppliers who offered guarantees over its origin.

The FSA is increasingly using DNA testing to establish the origins of food.

In 2003, the agency found that 46 per cent of basmati rice samples were bulked up with cheaper varieties-A follow-up survey in 2006 found 16 per cent of samples were still being adulterated.

Similar adulteration was found with arabica coffee, pure orange juice and extra virgin olive oil.

The FSA has set up a food fraud task force to target the menace.

Its chairman, Dr Philip Barlow, is calling for a more proactive approach to dealing with food fraud, more random testing, and for information on cases and prosecutions to be made available to the public.

"Food safety is of paramount importance and everyone expects that the food they obtain is of the highest quality, which in most cases it is," he said.

"However, there are always a few individuals who will, for personal gain, try to pass off suspect food as meeting the required standards."

Which? asked the big four supermarket chains what checks they carry out to prevent food fraud.

Asda, Morrisons, Sainsbury's and Tesco said they regularly audit and visit suppliers and also employ independent laboratories to test produce for authenticity.