Government departments have no idea how much they are paying for milk despite Whitehall criticising supermarkets and dairies over slashing payments to farmers.
Caterers buying milk for Whitehall canteens have refused to tell ministers how much suppliers of milk receive, a parliamentary answer has revealed.
Ministers repeatedly called for retailers and processors to play fair after farmers launched a wave of summer protests over the falling farmgate milk price.
The public sector, including schools, prisons and hospitals, buys huge volumes of milk but mystery surrounds how much ends up in the farmer's pocket.
Farming leaders, retailers and Opposition MPs lined up to criticise the Government for failing to lead by example.
Andrew Butler, acting regional director of the National Farmers' Union South West, said: "We expect all companies that buy dairy products to behave responsibly, including government departments.
"It's important that Government sets the right tone and ensures that the milk it buys at least meets the cost of production to dairy farmers."
Farming Minister David Heath responded to a written parliamentary question asking the Government to publish the price paid for milk. He said the Department for Environment (Defra) had written to all Whitehall departments to establish their procurement policies, but said: "Companies declined to say how much they pay their suppliers for milk on commercially sensitivity grounds."
The NFU estimates it costs 29p to produce a litre of milk, but farmers have been paid just 27p.
Defra, which is in charge of farming, says its catering company is committed to paying a "fair price".
A Defra spokesman said: "When Government buys food directly it uses public money without selling for profit, so has to strike a balance between support of the farming industry and responsibility to taxpayers. Defra is working with other departments to ensure all Government milk procurement is as transparent as possible."
Former Farming Minister Jim Paice was this summer left red-faced after admitting that he did not know what a pint of milk costs.
Huw Irranca-Davies, Labour's Shadow Farming Minister, said: "First we had a minister who didn't know the price of milk. Now we have a minister who is indifferent to whether central government is paying a decent price to our farmers."
Richard Dodd, British Retail Consortium spokesman, said: "It would be good if other big buyers of milk, including the public sector, were as transparent as supermarkets."