The hormone MPA - medroxyprogestrone acetate - was discovered in food additives which had originated in Ireland, and were sold on via Belgium.

All Dutch pig farms had to be temporarily closed when the contamination was first discovered last month.

Officials narrowed down the farms at risk to 37, where the slaughter operation is now taking place.

We have decided to look where blame can be laid, and that is with those who are responsible for introducing MPA into the food chain

Dutch agriculture organisation

Costs are estimated at around 8m euros, but farmers' groups say the real damages are likely to be much higher because of the countrywide closure.

Farmers say they may seek compensation from the companies involved.

"Costs will be in the tens of millions of euros at least, and probably over 100 million," said Jack Luiten, a spokesman for the Dutch Agricultural and Horticultural Organisation.

"We have decided to look where blame can be laid, and that is with those who are responsible for introducing MPA into the food chain."

Treacle

It is believed that the hormone originated in waste water from an Irish pharmaceutical firm, which was sold on to a waste management firm and then to Belgium.

By the time Dutch farmers bought it, it was contained in treacle.

It is not clear whether any of the firms realised that the substance was contaminated.

The slaughter operation is expected to last 10 days.

Last month, more than 300 farms in Germany were banned from selling animals or milk, after fears that they had also used animal feed contaminated with MPA.

The hormone is usually used in human contraceptives, or in hormone replacement therapy for menopausal women.