London-listed De La Rue, which has designed about a third of the world's banknotes in circulation and is the world's largest commercial printer of passports, said it was cooperating with the SFO and would provide a further update when appropriate.

"Given the early stage of these matters, it is not possible to predict reliably what effect their outcome may have on De La Rue," the company said in a statement on Tuesday. A spokesman declined to comment on the details or scope of the inquiry.

De La Rue, which secured the contract to design and print the new currency of South Sudan after the country seceded from the north in 2011 following decades of civil war, holds its annual shareholder meeting on July 25.

In a brief statement, the SFO confirmed the investigation into the company and "associated persons" and urged anyone with information to step forward. A spokeswoman declined to divulge further details.

South Sudan government officials were not immediately available for comment.

The investigation is a further setback for De La Rue, which last year lost the 400 million pound ($500 million) contract for Britain's new blue passports and has seen its stock tumble almost 30% since May after a profit warning, the departure of its CEO and a one-off hit for non-payment from Venezuela.

Shares in the company fell as much as 17.4% to 246 pence on Tuesday, hitting their lowest level since July 2003.

(Additional reporting by Kirstin Ridley in London, editing by Louise Heavens/Keith Weir and Emelia Sithole-Matarise)

By Justin George Varghese