The Farmers Union of Wales launched a campaign at the Royal Welsh Show this week encouraging visitors to send a specially printed postcard to Tony Blair, presenting key facts about the squeeze on dairy incomes.
A total of 2,000 of the postcards have been printed and designed with a fold-over pouch, which the FUW and the Women's Institute are asking people to put 6p into before posting.
A spokesman explained that 6p was the difference between the actual milk price, which averages little more than 18ppl, and that which is considered sustainable - 24ppl.
FUW president Gareth Vaughan said: "It's a scandal that dairy farmers are being paid less than the cost of production - and less than they were a decade ago - while the price of milk in the shops is unchanged.
"This poverty pay is threatening the existence of our dairy farmers."