Discount retailer Lidl is under fire from the National Farmers Union (NFU) for pressurising milk suppliers for lower prices.
According to the NFU, Lidl is attempting to force milk farmers to slash prices by between 1.5p and 3p per litre.
But the NFU argues that prevailing market forces mean the price of milk to retailers should be going up.
NFU chief dairy adviser Phil Hudson said that because the pound has weakened against the euro by 10 per cent in the past four months, there should be a substantial rise in milk prices. The EU has set a target of 22.5p a litre to be paid by retailers to farmers.
Hudson added: 'Lidl is the only retailer attempting to alter its position, after the 2p price increase (per litre) paid by retailers last autumn.
Market fundamentals suggest that prices should go the other way. We have been trying to get hold of Lidl to talk about this, but we haven't heard anything back.'
A Lidl spokeswoman refused to comment on the NFU's allegations.
Last January, Tesco came under fire from groups such as Farmers for Action for attempting to lower the price paid for milk to 18p per litre, when the minimum cost of production is about 21p.