Dairy farmers have reacted furiously to Asda's move to offer multibuy discounts on four-pint polybottles of milk.

The retailer's decision last week to offer two bottles for £2 - a 30p markdown on the normal price of £1.15 per bottle - needlessly devalued milk, they claimed.

One Asda milk producer wrote to The Grocer saying: "Please, Asda, what is your justification for this cut? If you say you are giving consumers more for their money, then I am completely lost! How about giving the farmers more for their milk?"

Asda claimed it was funding the price cuts from its own pocket - potentially costing it millions of pounds. But this news did not mollify the NFU, which abandoned its policy of discussing supply chain issues behind closed doors to condemn the move.

"We are disappointed Asda has decided to cut the price of milk," said NFU dairy board chairman Gwyn Jones. "It sends the wrong message to consumers about the value of milk."

The ongoing campaign by the Women's Institute and the NFU had shown that many consumers would pay more for their milk to support British dairy farmers, Jones claimed.

However, Asda dismissed farmers' concerns and said the promotion increased footfall in stores and boosted milk sales. Last year, a £1-a-bottle promotion lifted volumes by about 5%.

"The promotion will drive more volume," said a spokesman. "More milk sold means more farmers get the Asda 1p per litre bonus for their milk."

The promotion is backed by a new freshness guarantee, which states the milk will last seven days in customer fridges if it remains unopened. Milk monitors in stores have been trained to keep fresh product on the milk fixture well merchandised and reduce wastage.

Posters in-store also claim Asda is closer to its farmers than any other major multiple.