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 grandmother says she was outraged after her grandson was segregated from his classmates by a cardboard screen - for being one minute late.
Leaked documents have revealed the first known picture of Mohammed Emwazi - named as IS militant Jihadi John - as a student in London.
With March just around the corner it's looking like this winter will be the sunniest on record (data goes back to 1929).