The 43-year-old man, who runs a driving firm, is said to have sent threatening letters to the billionaire co-founder of software giant SAP since August.

Although details have until now been kept strictly under wraps, the letters were obviously taken seriously enough for the police to be notified. Nothing was made public on the case until the man had already been arrested.

This followed an agreement reached about paying him, and an undercover detective handing over a case of fake money at a meeting to which the alleged blackmailer showed up in one of his lorries.

A few days later when police raided his house in Heilbronn in early September, they found the fake money there, according to the case against him.

He was arrested and has been in police custody ever since.

Hopp, who earned his fortune from SAP, has invested heavily in the biotech industry involved in the search for cancer treatments, as well as his home town football team 1899 Hoffenheim.

Hopp has not commented on the case, and is not expected to be called as a witness in the trial which starts on Tuesday in the Heidelberg District Court. A verdict is expected on Wednesday.