A genetically modified (GM) variety of maize banned in the EU has been sown accidentally across Germany.
The NK603 variety has been planted in seven states. The seed supplier, US firm Pioneer Hi-Bred, called the level of contamination "minute".
It is not clear how the mistake occurred, but it could cost farmers millions of euros, as crops will now have to be destroyed.
The EU is currently reviewing its tight rules on the cultivation of GM crops.
Pioneer Hi-Bred, based in Buxtehude near Hamburg, says NK603 has been planted on "just under 2,000 hectares (4,940 acres)" of land. The environmental group Greenpeace put the area as high as 3,000 hectares.
Bavaria, Baden-Wuerttemberg and Lower Saxony are among the states where it has been sown.
Supporters of GM crops argue that they deliver higher yields and resistance to pests, requiring less fertiliser and pesticides.
Opponents say more scientific data is needed, arguing that their long-term genetic impact on humans and wildlife could be harmful.
They also say GM crops can enter the food chain inadvertently if they are naturally cross-pollinated with non-GM varieties.
Greenpeace says that officials knew about the contamination in early March, but that because of bureaucratic delays farmers are only now being warned.
"This is the biggest GM crop scandal in Germany to date," said a Greenpeace agriculture expert, Alexander Hissting.
Ploughing up fields
In the affected fields, up to 0.1% of the crop is contaminated with NK603 - equivalent to 100 contaminated plants per hectare, Greenpeace says.
Pioneer Hi-Bred disputes that figure. Company spokesman Mike Hall told the BBC that the level of NK603 detected in the "conventional seed" was 0.03%.
"It's highly unlikely that it's a GM trace. Anything below 0.1% could be a false positive, impossible to quantify scientifically," he said.
"In the past when they found trace amounts we removed the seed from the market. In this case they told us after it had been planted."
Stefanie Becker, spokeswoman for Lower Saxony's Environment Ministry, said that "fields will have to be ploughed up before the maize blooms - it is still possible to halt the uncontrolled spread [of the GM variety]".
She said her ministry did not get details about the distribution of the GM maize until last Friday. "We have the distributors' names, and through them the farmers will be informed," she told the BBC.
Ms Becker said the contamination affected about 2,000 hectares and originated from two sacks of seeds. It is not yet clear how the seeds got mixed up, she said.
So far the EU has allowed only two GM crops to be cultivated - Monsanto's MON 810 maize and a type of potato harvested for starch. But Germany, like some other EU countries, banned MON 810 last year.
EU member states are divided over GM crops. Commercial GM planting takes place in Spain, Portugal and the Czech Republic. But France, Germany, Austria and Greece are among several states that have banned MON 810.
The GM maize that has spread in Germany "is not harmful to human or animal health", Ms Becker said.
The European Commission is overhauling the rules on GM crops and will present new proposals next month allowing member states more freedom to allow or ban GM varieties.
Countries would be allowed to set their own technical standards for GM farming, including buffer zones to prevent cross-pollination.
The new rules will still require approval by EU governments and the European Parliament.