The widespread planting of GM crops in Britain could severely damage wildlife such as birds and insects, an expert scientific review will warn tomorrow.
An inquiry chaired by Professor Sir David King, the Prime Minister's chief scientific adviser, will endorse fears that growing some types of herbicide-resistant GM crops could have a significant impact on the countryside.
Some GM crops being considered for Britain, such as sugar beet and oilseed rape, are designed to survive the use of so-called "broad spectrum" herbicides that wipe out other weeds and plants.
But that would threaten wildlife, such as skylarks which feed on the "fat-hen" weed growing in sugar beet fields, creating the "green deserts" feared by many naturalists. "This is perhaps the most serious potential harm," the report says.
The warning will be one of the strongest conclusions from an exhaustive "science review" being published by Professor King tomorrow, which will also state that current GM foods are safe to eat.
Because of the perceived threat to wildlife, Professor King's committee will write an up-date report after the results of trials into GM maize, beet and oilseed rape come out in September.
The report also suggests that a new generation of weedkiller-resistant superweeds could be created in the future unless the Government and farming regulators are extremely careful about where and when different GM crops are used.
But it says this risk - based on the fear that GM genes which are resistant to different weedkillers could "stack up" in weeds - is only likely to arise in five to 10 years and only if GM crops become widespread in Britain.
At present, because few GM crops are likely to be planted in Britain due to public hostility, the risk of creating superweeds is seen as low.
But the document, agreed late last week by a panel of 24 leading scientists, biotech industry executives and naturalists, is likely to disappoint many anti-GM campaigners and environmentalists. It is understood to be more neutral and non-committal about the potential problems than a critical report on the economic value of GM crops released by the Prime Minister's Strategic Policy Unit earlier this month.
It will also say that GM crops should be planted in the UK, and licensed on a case-by-case basis.
The Downing Street document, which suggested there was little short-term economic or consumer advantage to planting GM crops, has led Tony Blair to substantially rethink his pro-GM stance. He is now said to be more cautious about supporting the commercial planting of GM crops in Britain.