The Serious Fraud Office is investigating the world’s biggest printer of banknotes over suspected corruption in South Sudan.

The SFO said it was examining how the UK company De La Rue, which produces notes for the Bank of England, and associated individuals conducted business in the African country.

De La Rue said it was cooperating with the SFO, which investigates serious fraud, bribery and corruption. The Basingstoke-based company and the SFO declined to give further details about the investigation.

“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.

De La Rue has done business with South Sudan since the country was established in 2011, designing and printing the country’s banknotes. The company produces notes for countries worldwide and is also the biggest commercial printer of passports.

The SFO’s attention adds to problems for De La Rue, which is looking for a new chief executive after Martin Sutherland agreed to leave following two profit warnings and the loss of a £490m contract to print the UK’s post-Brexit blue passport.

De La Rue shares closed down 15.8% at 251p on Tuesday. The shares have more than halved in the past year as the company has suffered poor trading and a series of setbacks.

The 206-year-old firm reported a 77% drop in annual profit in May and warned that profit would fall further this year as it faced tough competition. After losing out to a foreign rival to print the new UK passport, Sutherland said the decision risked jobs and that he would appeal, but gave up a month later.

De La Rue’s chairman, Philip Graham Rogerson, will step down once Sutherland’s replacement is found. The company’s senior independent director has also said he will quit by the end of 2019.

Rogerson and De La Rue’s board will face unhappy shareholders at its annual general meeting on Thursday. Last week the company rejected a call by its third-biggest shareholder, Crystal Amber, for Rogerson to stand down at the AGM. The activist investor has repeatedly criticised Rogerson’s performance.

The SFO has investigated De La Rue twice in the past 12 years. In 2010 it notified the SFO that some employees had falsified paper specification certificates, costing it at least £35m. Another investigation launched in 2007 was closed with no action taken against the company or employees.