Dover Harbour has been closed to swimmers and water-based activities after "above normal" levels of E. coli bacteria were found in the water.
The Port of Dover tweeted at about 21:30 BST on Saturday to say the decision had been taken after "routine water sample tests".
In a statement earlier, it revealed the concern about bacteria but said there were no reports of ill health.
"The health and safety of our customers is of utmost importance," it added.
Further water sampling will take place later and on Monday, it added.
In an earlier statement, the port said that when "water standards were mot met" it consulted with Dover District Council and the Environment Agency.
"The advice was to close the harbour to swimming and water-based activities until the situation is better understood, " it explained.
"The health and safety of our customers and community is of the utmost importance to us and therefore we have taken these measures as a precaution.
"There are no reports of ill health. We are working with external agencies in order to lift the restriction at the earliest opportunity."
Image copyright LKJ Image caption Sunbathers could be seen next to one of the signsIt apologised for the inconvenience and said further water sampling would be undertaken later and on Monday, with results expected by Tuesday.
However, local residents remain concerned.
Phil Smith complained about the limited information on Saturday night: "To give so little information to the people of Dover is an utter embarrassing shambles on your part Port Of Dover..."
Author Lady Kendall Jagger tweeted that signs along the beach were "not enough".