The email was intended for one departing employee, but was sent in error to the entire worldwide staff of 1,300.
The human resources department of the London-based firm quickly realised the mistake and recalled the message after 25 minutes. They then sent out an email apologising for the error, Yahoo reports.
The email did not tell recipients they were fired, but contained instructions for departing employees such as handing back company equipment.
Company spokesman Paul Lockstone said: "An email which was intended for a member of staff who was leaving today was accidentally sent to all Aviva Investors staff worldwide.
"People were pretty quickly aware of the fact that this was a mistake... I don't believe any of our staff would have seen it really as anything other than the mistake that it was."