Nigeria alleges that the money was used by the companies to buy an oil exploration license in the Gulf of Guinea but was diverted to bribes and kickbacks, the report said.

The government alleges that Shell and Eni were partly responsible for the fact that “corrupt Nigerian officials” used the money for personal enrichment, it added.

The alleged corruption is already the subject of a separate court case in Milan in which Nigeria is a civil party.

Shell and Eni have always denied any unlawful action.

In a statement, a Shell spokesman said that the deal in 2011 was “a fully legal transaction with Eni and the Federal Government of Nigeria”. “Since this matter is before the Tribunal of Milan, it would not be appropriate for us to comment in detail on the new claims that have been made,” it said.

A Milan judge in September sentenced two people considered intermediaries to four years in prison each.

The two were tried separately from the main trial involving Eni and Shell which opened earlier this year in Milan.

Nigerian President Muhammadu Buhari has put the fight against corruption at the heart of his bid for another four-year term ahead of elections next year.

Soon after coming into office, Buhari pledged to recover what he said were “mind-boggling” sums of public money stolen during previous administrations.

His anti-corruption campaign, however, has yet to secure any high-profile convictions while his opponents accuse him of mounting a political witch-hunt.