De La Rue's shares dropped more than 13% to 698 pence at 0949 GMT, after it said James Hussey
, who had worked for the firm for 25 years, resigned to "take responsibility" for problems "of a serious nature" at one its paper mills.
The company said in July that its paper factory in Overton in southern England suspended production of a special type of paper which is then printed elsewhere to make bank notes.
Rue is confident that neither the physical security nor the security features incorporated in the paper have been compromised for any customer," the statement said.
Hussey, 48, was managing director at De La Rue's currency division, before being appointed CEO in January 2009.
was made CEO after his predecessor, Leo Quinn, spun-off the company's cash systems business, which sold tills to retailers, to focus on making bank notes and products such as cheques, holographic marks and tax stamps.
was seen as an industry insider with strong international contacts, which meant he
was well placed to maintain the company's relationships with clients around the world, Jones said.
"You're losing someone with a wealth of experience, someone with a lot of contacts and in this industry it's all about contacts," he
"Its a very unusual business, it's not a business with any structural problems, you don't need a company doctor you need an industry man and (Hussey's) an industry man," he