Hundreds of thousands of organic chickens in Germany are to be slaughtered after animal health inspectors found they had been fed meal contaminated with a banned herbicide.

The scare is being heralded as the largest ever food scare to hit the organic farming sector. The chickens were fed feed contaminated with a chemical called Nitrofen.

Nitrofen is banned in the European Union because it is believed to be carcinogenic. The herbicide has been traced back to North German grain distributor GS Agri, which sold 600 tonnes of contaminated wheat to over 100 organic farms around Germany.

Some animals on conventional farms may also have eaten the contaminated feed.

 German Agriculture Minister Renate Künast said it was still not known how the contamination occurred, and Lower Saxony state's agriculture minister Uwe Bartels said sabotage could not be ruled out.

"There was an effort to hide the contamination," Kuenast was quoted as telling the Tagesspiegel newspaper in an article released ahead of publication on Friday.

Food retailers including Edeka, Wal-Mart, Real, Extra and Kaufhof have removed organic food from their shelves and authorities have impounded more than a quarter of a million animals, mainly chickens, on the organic farms which received tainted feed. Neighbouring countries have banned imports of German poultry.

Meat and egg samples taken from farms tested positive and some were 18 times over the danger levels, reported Reuters.