Sophos has been ordered to pay $15m in damages after being found to have five patents owned by cybersecurity vendor Finjan.
The verdict, confirmed by both Sophos and Finjan, followed a two-week trial in California.
New York-based Finjan, which specialises in enterprise web security, had originally claimed that six patents were infringed.
In a statement to investors Sophos said: "This verdict represents the next step in an ongoing legal process and Sophos is considering all options including an appeal of the verdict."
Finjan also has pending infringement lawsuits against FireEye, Symantec, Palo Alto Networks and ESET relating to over 20 patents.
At the end of last year Sophos settled a long-standing legal battle with Fortinet and agreed to make a one-off payment.
In August, Finjan reported its revenue for the three months ending 30 June 2016 increased nearly 10-fold year on year to $6.5m.
Earlier this year Finjan also settled its litigation with Proofpoint for $10.9m.
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