The Irish Dairy Board (IDB) has cut returns for December milk supplies by the equivalent of 0.6c/l.
In a worrying move for farmers, the board reduced the price paid for butter last month by €150/t to €3,400/t.
While demand for butter has been weaker over the past few months, it had been expected that prices would hold for the early part of the year. However, the IDB cut has the potential to undermine milk prices.
The move has provoked a strong reaction from farm representatives.
ICMSA deputy president Pat McCormack described the IDB decision as "unjustifiable" and "at variance with recent market movements".
But a spokesman for the IDB pointed out that global milk production increased by 2.5pc last year, while butter markets had been weaker since September. He said the latest adjustment in returns to co-ops reflected these market developments.
Reports of EU butter imports from New Zealand are said to have unsettled the trade.
Meanwhile, milk supplies in November fell dramatically compared to last year, according to the latest CSO figures.
Volumes were back by nearly 240m litres (over 20pc).
However, milk supplies for the year are still up 5pc on 2011. Milk supplies across the EU are also up for the first three quarters of the year, driven by increases of up to 7pc in France, Germany and Britain.
The latest US forecast on global milk production shows Argentina should grow 4pc, the US by 1pc and New Zealand by 2pc.