New Zealand First leader Winston Peters has doubled down on his criticism towards Fonterra's plan to sell Mainland and Anchor to the French company Lactalis. 

In an open letter to the company Peters threatened new regulations if the sale goes ahead.

ACT leader David Seymour hit back saying the decision should be up to the farmers, not the Government, a sentiment echoed by Prime Minister Christopher Luxon on Monday. 

Peter's claims the letter was not a threat and he did not intend to interfere with the farming community or Fonterra.

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