Chip shop and takeaway owners across Norfolk face six months in jail and fines of up to GBP5,000 if they use genetically modified ingredients and fail to tell customers.

Norfolk trading standards officers surveyed independent caterers across the county including chip shops, sandwich bars, restaurants and private members' clubs and found that customers were eating genetically modified foods whether they wanted to or not.

While caterers are free to use GM ingredients, the law says that they must declare it to consumers.

Conrad Meehan, senior trading standards officer at Norfolk County Council, said a labels spot-check of ingredients used by independent businesses found GM products in soya and vegetable oils and mayonnaise.

'From speaking to these businesses it became clear that they really haven't got a clue that they had to declare it and in most cases they haven't realised that they were using GM food,' he said. 'It's a case of ignorance rather than trying to pull the wool over people's eyes, but that's certainly no excuse.

'It's a serious offence,' he added. 'The approach is if we carry out an inspection and we spot it we will give advice. But if someone fails to follow that advice then we will consider more formal action.

Mr Meehan added. 'We are hoping that consumers will start asking the question, which will prompt businesses to get it right. We will consider a further survey later this year to see if the publicity has had an effect.'

Bob Fox, from the Nationwide Caterers Association, said he was not surprised at the findings.

'We try and give as much advice as we can,' he said. 'The trouble is there is an awful lot of confusion about GM, not only within the trade but in the public's minds as well. I don't think anybody really understands it at all.'

Ian Gibson, Norwich North MP and a member of the House of Commons science and technology select committee, said that the government needed to clarify the rules on GM ingredients.

'It's being slipped through,' he said. 'There are some people who want GM foods [??? Who exactly?] and others who don't - but they should at least be given a choice, and consumers need the knowledge to make that choice.'

Trading standards officers visited 50 catering establishments in the county during late summer 2007 and early this year, to determine the extent to which GM ingredients are being used to prepare meals and whether those ingredients are being declared to customers.

Of the 50, 21 (42pc of the total) were using either vegetable oil or mayonnaise which contained oil from genetically modified soya beans and have subsequently been issued advice on their legal requirements by Norfolk County Council.

The labelling requirements are set out in EC Regulation 1829/2003 which states that where food is offered for sale to consumers and contains GM ingredients, it must be labelled as genetically modified. For caterers, this information would normally need to be given on the menu.

Jennifer Parkhouse, from Norwich Friends of the Earth, said: !The consumer doesn't want GM; that's been clear for a number of years and they would be horrified to realise they were eating GM food without realising it.'

An information sheet for businesses about this issue is available from Norfolk County Council Trading Standards by phoning 0844 800 8013.