A jury at Chichester Crown Court found the water company guilty of pumping raw sewage into the sea in September 2012, causing ten miles of beach to be closed for days between Ferring and Lancing. The company will be sentenced at a later date.
During the week-long trial, Southern Water denied three charges of breaching environmental permits relating to the incident in 2012, which was caused by three pumps failing at East Worthing wastewater treatment works.
Last week, the company was cleared of two charges that related to the proper maintenance of equipment.
The Environment Agency told the court that a slick of pollution was visible within hours of the untreated sewage being pumped into the sea.
Southern Water said it had activated its emergency plan to discharge sewage into the sea to prevent it backing up in the sewer network and flooding properties, including Worthing hospital.
Following the verdict, the company apologised to customers and businesses affected by the incident. Southern Water added that it had since spent around £20 million on improvements at the treatment works.
A version of this article first appeared on WWTonline.