In a brief announcement the SFO said: “The SFO confirms it has opened an investigation into the activities of the De La Rue group and its associated persons in relation to suspected corruption in South Sudan.”

In response De La Rue issued a statement saying that the SFO had informed the company about the investigation. “De La Rue intends to co-operate with the SFO in its investigation,” it said. “Given the early stage of these matters, it is not possible to predict reliably what effect their outcome may have on De La Rue.”

De La Rue said it would provide a further update “as and when appropriate”.

De La Rue’s share price fell by 15.7% to a new five-year low of 250p on the news. The group continues to be viewed as a potential takeover target.

No further details were forthcoming from either the SFO or De La Rue and the nature of the activities being investigated has not been specified.

South Sudan became independent from Sudan in July 2011. At the time De La Rue pulled out all the stops to plan, design and print a new range of currency for the fledgling nation in just six months, a third of the time usually taken for a project of that nature.

De La Rue’s AGM takes place tomorrow (25 July). Richard Bernstein’s Crystal Amber Fund has called on chairman Philip Rogerson to step down, and said it will call for an Extraordinary General Meeting if he doesn’t.

De La Rue chief executive Martin Sutherland has already announced that he will step down. There is no news as yet on his successor.

De La Rue’s share price had slipped further, to 245.5p, in early trading this morning.

The SFO said that it encouraged anyone with any relevant information to make contact through its secure reporting portal.