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French billionaire Vincent Bolloré has been placed under formal investigation by a French judge in a corruption probe.

A Bolloré Group subsidiary allegedly undercharged for work helping two African presidents win power in return for lucrative contracts.

In a statement, the company said he would now have the chance to address these "unfounded accusations".

Mr Bolloré was detained for questioning on Tuesday. He has been released.

The developments come days after he stepped down as chairman of the media group Vivendi.

Investigators are looking into allegations that his Havas advertising agency - a subsidiary of the group - provided discounted communications advice to Guinean President Alpha Condé and Togolese President Faure Gnassingbé during elections in 2009 and 2010.

In return, the Bolloré Africa Logistics company was allegedly given licences to operate container ports in Conakry and Lomé.

All parties deny the allegations.

In a statement quoted by the Reuters news agency, the Bolloré Group said that Vincent Bolloré remains presumed innocent, will now be able to see a dossier of evidence, and "will have the opportunity to answer these unfounded accusations".

Two rival firms have taken the Bolloré Group to court over the port contracts.

and "renowned expertise" were already sufficient grounds for it to have been awarded the licences.

Mr Bolloré's business empire includes shipping, advertising, construction and media and has large interests in Africa. He is one of France's best-known businessmen.

After the election in Guinea, Mr Condé terminated the contract of Conakry port's existing operator and gave it to the Bolloré Group.

A spokesman for the government of Guinea said there was nothing wrong with the contract for the shipping container terminal.