The latest estimate from the British Potato Council puts producer-held potato stocks at 3.36m tonnes at the end of November.
It represents a modest fall compared to last year, but more worrying for growers is that utilisation has also fallen sharply.
The BPC estimates that 2.43m tonnes had been sold from stores in the five months to the end of November, down nearly 9% on the same period in 2004.
Denis Alder at the BPC said blamed weaker demand for processed potatoes in the retail sector and weaker sales to foodservice operators.
Sales of fresh potatoes in supermarkets were as strong as ever, he added.