Brighton's central beach improved its rating to "recommended"
Rapid action is needed to deal with sewage being discharged on to a Sussex beach, according to a report on the country's bathing water quality.
The Marine Conservation Society (MCS) has warned that the problems at Brighton's Portobello beach need to be "urgently rectified".
Its annual Good Beach Guide also shows the number of beaches in Sussex with excellent water quality fell slightly.
Southern Water said it was looking for a new site for sewage treatment works.
A spokesperson for the company said plans to upgrade the Portobello waste water site, the only one in the region not to meet European standards, were refused planning permission in 2001.
A proposal for an alternative works at Peacehaven was rejected by the government in 2007 but plans for a more environmentally "sympathetic" design are being considered.
The MCS also said Sussex beaches needed measures to reduce run-off from storm pollution.
The society blamed the drop in results on last summer's heavy rain for sweeping raw sewage and animal waste into rivers and the sea.
This year 17 of 30 Sussex beaches were given an excellent rating, compared to 22 out of 33 in 2007.
The tests, carried out between May and September last year, coincided with what was thought to be the wettest British summer on record.
Thomas Bell, MCS coastal pollution officer, said: "These latest MCS results buck the long term trend of cleaner bathing water but we're pinning the blame squarely on last summer's exceptionally bad weather.
"Not all beaches are affected but the problem for swimmers is knowing when and where this has happened."
He advised bathers to choose beaches with a good record and stay out of the sea for at least 24 hours after heavy storms.
This year's report rates beaches from recommended - with the "highest water quality standard and good continuous sewage discharges" - to a basic pass, which shows the water quality reaches the minimum legal standard.
Cooden Beach, in East Sussex, failed the water quality tests.
Littlehampton and Brighton Central had the strongest improvement from 2007, dropping their "basic pass" ratings for the coveted "recommended" standard this year.
However, Middleton-on-Sea, Worthing and South Lancing in West Sussex and Brighton Kempton, Peens Bay and Norman's Bay in East Sussex had their ratings reduced from "recommended" to "basic".