The business blamed the sales drop on a slump in supermarket orders, and also said it was also partly due to supplies being disrupted by problems at the firm's new depot in Derbyshire.
Last year, the company made £7.5m in pre-tax profits and in October told its investors it expected to make annual profits of nearly £10m for the year 2014-15.
In a statement, Thorntons said: “We have recently experienced a significant reduction in previously indicated orders from the major grocers who also took in stock later than anticipated. While there has been an overall decline, the performance in the grocers has been mixed with good growth in several of our major partners yet significant volume decline in some others where prior year sales of high-volume lines have not been repeated.”
Despite these problems, sales in Thorntons' own shops and in convenience stores have been growing.