Shares in the world's largest banknote printer plummeted on the news, dropping more than 12% to £6.93 at the time of writing.

Hussey's resignation comes just three weeks after the announcement of "quality and production irregularities" at the Overton mill, which the firm said would lead to "materially lower" paper sales in 2010/11.

De La Rue issued a further statement this morning (12 August) announcing Hussey's departure, in which it stated that the production problems were "of a serious nature" and were still under investigation.

It said: "De La Rue advises that James Hussey has today resigned from the board as CEO and has left the company. Mr Hussey's decision to resign follows the company's statement of 20 July 2010 in relation to certain quality and production irregularities at one of its paper production facilities and his belief that he must take responsibility for this.

"The company continues to look into these irregularities in one of its paper production facilities, which the board considers to be of a serious nature.  A further announcement will be made as soon as the board has been able to assess the financial and operational implications on the current financial year's results and on the group's prospects but, owing to the complexity of the issues, this is unlikely to be in the near future."

Following Hussey's departure, non-executive chairman Nicholas Brookes has been appointed executive chairman, while group finance director Colin Child has been given the additional position of chief operating officer, until a new chief executive can be found.

The company added that it was confident that "neither the physical security nor the security features incorporated in the paper" had been compromised and that the irregularities relate only to "testing of paper specifications" at Overton.

De La Rue stressed that its currency printing operations were unaffected by the stoppage at its Overton plant. Meanwhile, the trade union Unite said that its members were being briefed by the company on the situation.

Unite assistant general secretary Tony Burke said: "De La Rue has notified us of this decision, which came as a complete surprise. The De La Rue management is now briefing Unite union reps and employees through their normal structures, which we would expect them to do. Equally De La Rue management are keeping us updated with developments in this regard."

Read associate editor Jo Francis's blog: Drama at De La Rue