A man dubbed the Italian Casalesi mafia's "finance minister" has turned supergrass, in a move an expert says will "make the political and business world tremble".

Antonio Iovine has begun talking to investigators about his role in the branch of the Camorra mafia in Caserta, southern Italy, reported La Repubblica.

Iovine spent 14 years on the run before being arrested in 2010, and is currently under witness protection.

"The collaboration of the Camorra boss Antonio Iovine with police will make the Italian political and business world tremble," Roberto Saviano wrote in the paper.

Saviano is himself in a witness protection scheme after his book, Gomorra, detailed the inner workings of the Camorra and earnt him death threats from the Casalesi.

He said Iovine's decision to turn state witness was unprecedented in the Camorra mafia and "could change everything".

According to a 2013 Europol report, the Casalesi clan was involved in crimes including cocaine trafficking, laundering money through construction firms, and infiltrating public funds.

Experts believe that Iovine's senior role in the clan means he would have dealt directly with corrupt politicians, officials and business owners. Saviano predicted that Nicola Cosentino, key ally of former premier Silvio Berlusconi in the Campania region around Naples, could be among those whose secret dealings are about to be laid bare.

"The news risks changing the relationship between business people and organised crime forever; not only in Campania [region], not only in Italy.

"He knows everything," Saviano said.

The 49-year-old Iovine was sentenced to 20 years in jail in absentia for his crimes while on the run.

Three other clan chiefs were jailed in 2008, but Iovine is the first to turn state witness.

Early in May, Iovine at last began talking to prosecutors, and was transferred from Sardinia's notorious Nuoro prison into a witness protection scheme.

His wife, Enrichetta Avallone, and 25-year-old old son have also been placed in the programme.