The Serious Fraud Office (SFO) said that instead of returning thousands of pages of evidence to 59 different sources, it sent the entire batch of material to a single recipient.
The office said it had already contacted the 59 people involved and that it is "working to contact any others who may have been affected".
More top news
A quiet and calm end to month and staying frost free tonight.
The FBI is reopening an investigation into e-mails sent by Hillary Clinton via a personal server, its director has announced.
Former deputy prime minister is set to return to the Lib Dem's front bench, the Party's leader has confirmed.