The daughter of Congo Brazzaville's President Denis Sassou Nguesso has been charged with corruption by French authorities.

Julienne Sassou Nguesso, 50, and her husband Guy Johnson, 53, were placed under investigation earlier this month for "money laundering and misuse of public funds."

French authorities were trying to determine how the coupled had managed to buy a €3m (£2.7m) mansion in the Paris suburb of Neuilly-sur-Seine in 2006, a judicial source told AFP.

Nguesso is an insurance agent and her husband works as a lawyer. The couple spent €5.34m to renovate their seven-bedroom house between 2007 and 2011, raising the total investment in the mansion to nearly €10m.

Investigators suspect the couple may have financed part of the project through an offshore company in the Seychelles and through sales of shares Julienne Sassou Nguesso owned in a telecommunications company that was linked to "corruption operations", the source explaied.

The couple has not commented on the investigation. However, Jean-Marie Viala, the Nguesso family lawyer, was quoted by AFP as saying: "This affair that has been going on for 10 years will be dismissed through totally legal procedures."

Wider investigation

The investigation is part of a wider probe by the French police into how members of the families of three African presidents - Congo, Gabon and Equatorial Guinea - have spent their money in France. The investigation started in 2010.

Nguesso 's husband is also under investigation for his alleged role as asset manager of a real state company that bought a €19m mansion in another upmarket area of Paris in 2007.

Gabon's late president Omar Bongo and his late wife Ondimba, Sassour's oldest daughter, owned shares in the company.

In 2012, French authorities seized a mansion in Paris belonging to Teodorin Nguema Obiang Mangue – son of Equatorial Guinea's President, Teodoro Obiang Nguema Mbasogo – and issued an arrest warrant for the president's son on money-laundering charges.

The 48-year-old man is suspected of having used state funds to buy a six-storey mansion in Paris' Avenue Foch as well as a collection of Italian super cars. Obian has denied the charges and claims the money he used came from legitimate sources.