Sewage spills into seas and rivers by Southern Water nearly doubled in a year.
There were 29,494 spills last year by company which supplies water to most of West Sussex as well as Brighton, Hove, Lewes and Hastings.
This was up from 16,688 the previous year.
The water giant's sewage spills lasted for a total of 317,285 hours in 2023, equivalent to more than 36 years.
Water UK, the industry body for sewerage companies, said the rise in spills across the country was "unacceptable" but the record levels were due to heavy rain.
Sewage spills are not illegal but the Environment Agency said: "It is important to note that heavy rainfall does not affect water companies' responsibility to manage storm overflows in line with legal requirements."
Southern Water, which also provides sewage treatment for Chichester, Bognor and much of East Sussex, said reducing storm overflows is its top priority.
John Penicud, director for wastewater operations at the firm, said: “Slashing the number of storm releases is top priority for us - and our customers. Last November we announced our £1.5 billion storm overflow reduction plan which will combine innovative engineering with nature-based solutions.
“The past 18 months have been the rainiest since records began. The ground is utterly waterlogged in many areas, inundating our own sewers and customers’ drains and sewers.
“We’re extensively relining sewers to keep sewage in and rainwater out and our storm release reduction pilot schemes have already proved that nature-based systems can have a real impact.”
The average time a sewage spill lasted rose from 8.8 hours in 2022 to 10.8 hours last year.
It comes after the company's boss admitted problems with the sewer network will take years to fix.
In January, Lawrence Gosden, chief executive officer of Southern Water, told The Argus: "I want to protect the natural environment. I've come into this job to change the company and also to put it on a path to be able to deal with overflows.
"But I think the point that I'm maybe not making clearly enough is just the scale of the infrastructure. This is not an overnight fix."
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