Written threats to contaminate New Zealand infant formula with pesticide amount to “eco-terrorism”, says Prime Minister John Key.

However, police believe the risk of it happening is low.

Anonymous letters were sent to dairy giant Fonterra and national lobby group Federated Farmers in November last year threatening to put 1080 in infant formula unless the government banned the agricultural pesticide by the end of March, it was revealed on Tuesday.

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The letters also contained actual formula laced with a fatal amount of the pesticide.

Police said they were investigating the threats even though they believed the risk of anyone carrying them out was low.

Key labelled the campaign “eco-terrorism” and insisted he would not be negotiating with those responsible.

“The person’s motive is to try and bully the government to stop using 1080,” he said.

“If you start negotiating with eco-terrorists, then that’s a technique for any particular issue that they don’t like about the government.”

A sign warning supermarket customers of increased in-store security in Auckland. Photo: Getty

Ministry for Primary Industries – which has called the letters “blackmail” – has done over 40,000 tests over the past few weeks and found no trace of 1080.

Dairy companies Fonterra, Synlait and Tatua have criticised the threats but have reassured consumers their products are safe.

“It’s wrong that someone will have a crack at reasonably defenceless people, so of course there’s an element of worry there,” Paul McGilvary of the Tatua dairy co-operative said.

“But we believe that our supply chains are absolutely locked down, and we believe that the police will find this person.”

Security has ramped up at supermarkets, with the Countdown chain moving its infant formula behind counters and competitor Foodstuffs restricting shoppers to two tins per product line.

Both companies are asking customers to check their infant formula for any sign of tampering.

International trading partners first learnt of the threats in February.

It’s still unclear why it’s taken so long to make the threat public, although Key says police needed more time to investigate before the threat was in the public domain.

The Department of Conservation uses 1080 to protect native species and reduce the risk of bovine tuberculosis from infected possums.

The poison’s use is strongly opposed by some recreational hunters and pet owners who fear their dogs may eat it and die, and by some environmental activists who believe 1080 represents a longer term risk in the natural environment and waterways.

However, its supporters include the Royal Forest and Bird Protection Society, and Parliamentary Commissioner for the Environment.

-AAP