The survey, carried out by the British Free Range Egg Producers Association (BFREPA) and funded by the Ministry of Agriculture and the Universities Federation for Animal Welfare, covers nearly three-quarters of Britain's 300 free-range egg producers.
It will be published in full at the end of November, but early results show that some farmers operate large single sheds containing up to 35,000 birds which restrict their freedom to move outside - either by providing too few exits or by being located too close to tall cereal crops, which intimidate hens.
The BFREPA has written to the big supermarkets to warn of these 'highly undesirable' trends.
'No one likes to be duped or made a fool of,' says the association's letter, a copy of which has been seen by the Independent. 'Should the consumer become aware of the circumstances we consider questions will be asked to the embarrassment and cost of all those involved.'
The BFREPA's code of conduct, not legally binding, says no single building should house more than 10,000 birds. The law says producers or eggs bearing the words 'free range' must give each hen at least 10 square metres of free ground, but it places no limit on the number of birds any building can hold.
The association is intending to use the survey to help raise industry standards to the level of its code, and is lobbying Gillian Shephard, the Minister of Agriculture, and the ministry's Egg Inspectorate. It wants the law to specify that free-range henhouses should adjoin pasture and that they should have an adequate number of exits.
But it wants the big supermarkets, which sell 90 per cent of the United Kingdom's free-range eggs, to tighten up voluntarily first.
The supermarkets deny they are being misled. A spokeswoman for Sainsbury's said its 'tight specification' to suppliers ensures its free-range hens can range over pasture: 'We make visits both announced and unannounced to our suppliers and packhouses. If we discovered that a supplier was trying to hoodwink us, we would cease to deal with them.'