The RSPCA is urging shoppers to avoid cut-price milk sold by supermarkets, warning that it could have a knock-on effect on cow welfare.
Recent price cuts threatened to badly affect the welfare of dairy cows, said the charity, urging shoppers to avoid tempting milk deals.
Although some supermarkets pay farmers for milk based on the cost of production (COP), the RSPCA said most diary farmers were not lucky enough to be on such contracts.
Farmers not on COP pricing models are frequently receiving less than 25p for milk that costs them more than 30p/litre to produce.
Major processors have slashed some 4p a litre off the farmgate milk price since 1 April.
Producing milk at a loss was unsustainable, said John Avizienius, deputy head of the RSPCA's farm animal science team and a former dairy herdsman.
"Although a drop in cost of milk and cheap deals might seem like great news for shoppers, we are concerned that ultimately it will be cows that will pay the price.
"We are in the ridiculous situation where milk is cheaper than bottled water at some supermarkets; that simply cannot be right."
Cows would ultimately pay the price for cheap milk, said Mr Avizienius.
"We know people care about the welfare of farm animals and would urge them to think of dairy cows when they are doing their weekly shop.
"I believe shoppers would be even happy to pay an extra one or two pence on a pint of milk if it safeguarded dairy cow welfare."
The RSPCA had been working on a number of initiatives and projects with the government and the dairy industry to improve welfare in the UK.
Farmers were making efforts to tackle important issues such as mastitis and lameness in the national herd through the national dairy cow welfare strategy, which the RSPCA was helping to implement.
"I am concerned that this drop in the market price of milk could put the brakes on these and other planned welfare improvements.
"For example if a farmer was due to upgrade their cubicles or straw yards to make them more comfortable for their cows they may not have the money to invest in that now."
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