The National Council of the Irish Creamery Milk Suppliers Association has unanimously decided to back a Yes vote in the Lisbon Treaty referendum.
It follows a call from the association's president John Cahill to vote in favour of the Treaty, after his talks with the Taoiseach at the weekend.
Mr Cahill says he received an "iron-clad" assurance from Brian Cowen that he would use his veto to protect Irish farmers in the ongoing WTO talks.
The country's second largest farming association made the decision to support the Treaty at a meeting in Limerick today.