Earlier results showed contaminated milk in only two of the closed farms, the ministry has said.

The dioxin scare has spread to Germany, Belgium, France and Spain in the past two weeks but the European Union and Dutch health officials have ruled out any risk to public health, saying contaminated products had not reached consumers.

Some 100 Dutch pig, cattle, sheep and goat farms will remain shut until all tests are completed in the second half of next week, the ministry said.

Dutch authorities have said that German-made clay used for sorting potatoes was the reason for the dioxin contamination of animal feed made using potato peelings.

The clay was bought by 10 Dutch potato companies and was also shipped to companies in France and Belgium, the ministry has said.

It has said that two cows from one of the affected Dutch farms were exported to Spain.

Dutch authorities have also found that contaminated feed had been exported to Belgium and Germany, which has led to the temporary closure of eight livestock farms in Belgium and five in Germany.