A food crisis in Germany deepened Friday as around 4,700 farms were closed after tests showed animal feed was contaminated by a cancer-causing chemical, and officials said they suspected foul play.

Fears also grew that the contamination could have entered the food chain earlier than thought, as tests on animal fats at the firm at the centre of the scandal reportedly showed they were tainted as far back as last March.

A spokesman for Agriculture Minister Ilse Aigner told a news conference Friday that "4,709 farms and businesses are currently closed," including 4,468 in the state of Lower Saxony, northwest Germany.

The farms will be closed until they are found to be clear of contamination with dioxin, a toxic chemical compound that can cause cancer if ingested in large doses, and will not be allowed to make any deliveries, spokesman Holger Eichele said.

Nearly all types of farms, especially those rearing pigs, have been affected by the closures in eight of Germany's 16 states, the agricuture ministry said. There are around 375,000 farms in Germany.

The firm Harles und Jentzsch in the northern state of Schleswig-Holstein is alleged to have supplied up to 3,000 tonnes of contaminated fatty acids meant only for industrial use to around 25 animal feed makers.

Most of this -- 2,500 tonnes -- was delivered in November and December to animal feed producers in Lower Saxony, where it was used in fodder.

Tests on samples from Harles und Jentzsch showed nine samples out of 20 had dioxin levels higher, or much higher, than permitted, the Schleswig-Holstein ministry said on Thursday.

"The first indications point to a high level of illegal activity," said Eichele. Prosecutors have launched an investigation into the company.

The vice president of the association for consumer protection and food safety in Lower Saxony, Konrad Scholz, told the Tagesspiegel daily: "At such high levels, it cannot just be a mistake."

Meanwhile, German farmers were left to count the cost.

The head of the country's farming association, Gerd Sonnleitner, told the Neue Osnabruecker Zeitung regional daily on Friday: "One has to assume that farmers will be losing around 40 to 60 million euros per week."

However, Eichele said it was "not yet possible to judge how large the economic damage will be."

The Hannoversche Allgemeine Zeitung reported earlier Friday that tests conducted on Harles und Jentzsch as far back as March last year had revealed dioxin levels twice the permitted maximum amount.

However, the March test results did not come to light until late December, the paper said.

The scandal has also spread beyond Germany's borders.

German authorities on Wednesday told the European Commission and business partners that 136,000 eggs from suspect farms had been exported to the Netherlands.

And the European Commission said Thursday the hunt had also turned to Britain but the country's Food Standards Agency (FSA) said the tainted eggs were not thought to pose a threat.

The German government said earlier that up to 150,000 tonnes of feed overall were feared to have been contaminated. Some 100,00 eggs have been destroyed as a precaution in Lower Saxony.

Dioxin, a by-product of burning rubbish and industrial activities, can cause miscarriages and other health problems in humans, including cancer if consumed in high levels.

Despite officials insisting there is no health risk, Germans appear to be already altering their purchasing habits, said Margit Beck from a chicken and egg market research firm.

There has been a "clearly noticeable" impact on the market already, Beck said, adding it was too early to give precise figures.

"There is a lot of uncertainty, both from consumers and from producers," she told AFP.

However, a survey conducted for ARD public television suggested two thirds of German consumers would not change their egg consumption.

Only 14 percent of people said they would eat fewer eggs and four percent said they would shun eggs completely.

One German pensioner, who declined to give his name, seemed clear on the subject, however. "Dioxin is a very dangerous substance. I've just discussed it with my wife. I have 10 eggs at home and I'm not going to eat them now."

"We're going to have to do without eggs now," he told AFP.