The ship canal, built in 1890s, stretches 36 miles (57km) from the Mersey Estuary to Salford Quays.
There are around United Utilities’ 100 outfalls along the canal, where treated waste is released from the sewerage network.
Raw sewage is also dumped at times when the system is operating over capacity, something the Supreme Court judges said could be avoided if the firm “invested in improved infrastructure and treatment”.
They found the canal company was entitled to bring a nuisance or trespass claim for these incidents, even in instances when there had been “no negligence or deliberate misconduct” by United Utilities.
It comes despite rulings to the contrary in both the High Court and the Court of Appeal, which led to the canal company’s appeal.
A Peel Ports spokeswoman said the decision gives the watercourse owners “the right to bring legal action against a water company that discharges pollution into its watercourse”.
Andrew Ross of law firm Charles Russell Speechlys, said the Supreme Court’s decision could lead to “an increase in this type of legal claim”.
“Owners of waterways may now consider taking action against sewerage undertakers for polluting water,” he added.
United Utilities said it “understood and shared” concerns about the need to improve, and pointed to a £3 billion investment plan aimed at cutting pollution by improving infrastructure.
