Swimmers have been advised to stay out of the sea at a Clacton beach because of bacteria including E. coli in the water.
The Environment Agency said it had found good or excellent quality of water across many beaches in Tendring following extensive testing.
However, that next to Clacton Pier at Groyne 41 was once again poor with high levels of Intestinal Enterococci and E. coli, an issue which has been affecting the area since 2011.
• See the water quality profile at your nearest beach hereInvestigators said they are not certain what is causing the problem and are carrying out further tests.
John Daniels, senior environment officer, said: “Tendring’s bathing waters were generally of a very high standard, with excellent and good ratings for most beaches.
“Samples are taken regularly throughout the bathing water season and this enables us to check they are safe for people to enjoy. We know how much people like going to the beach and the great benefit this has on the local economy. We will continue our work to ensure these standards are upheld.
“We are working closely with partners to try and establish the cause of bathing water quality issues at Clacton. Investigations have led us to believe that contaminated groundwater is the most likely source of the problem.
“We will continue to investigate the pollution sources at the beach to help improve water quality.”
Environment Agency officers visit each bathing beach from May to September every year to see what levels of bacteria are present in the water. Officers use results to maintain and improve bathing water quality.
Problems with water contamination at Clacton Pier first materialised in 2011 and the sewage system was monitored with CCTV before being discounted as the cause.
The results greatly improved in 2013 before once again being flagged up the following year. They have since persisted, with contamination found two metres below the beach surface also.
Pigeons roosting under the pier have previously been blamed, but while it is thought they may still play a part, it is now thought to only be a minor cause.
New investigations will focus on the groundwater at the beach, which is now considered to be the culprit, in an effort to resolve the problem for next year.
Environment Agency investigators said they will be taking new samples in the coming days at Clacton Pier, and have been working with partners such as water companies to address the issue.
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