It has never been entirely clear how much Mr Werner is master-spy, and how much master self-publicist.

A freelance agent rather than a government intelligence officer, he started out as a private detective, founding his own agency at the age of 20.

He is believed to have worked extensively undercover for Germany’s BND intelligence service and the Bundeskriminalamt, the country’s national police CID.

He claims to have also worked for various other Western intelligence agencies, but the shadowy nature of the work makes the details impossible to confirm.

The case against him centres on various offshore accounts he allegedly holds in the Bahamas, Luxembourg and elsewhere.

Proscutors say he failed to disclose the existence of the accounts to the German tax authorities.

His name has aslo been connected with various offshore shell companies listed in the “Panama Papers”.

Lawyers for Mr Werner have contended the accounts were set up by various intelligence agencies and that he used the money for operations including hostage releases.

After the investigation was opened into him in 2012, Mr Werner’s lawyers reportedly wrote to prosecutors suggesting they “set aside customary and recognized rules of evidence in this case and recognize that they have encountered the borders of the secret sphere unknown to standard state agencies”.