Dover Harbour has been closed to swimmers and water-based activities as “above normal” levels of E. coli bacteria have been found in the water.
The Port of Dover tweeted at about 21:30 BST on Saturday to say the closure decision had been taken following “routine water sample tests”.
Customers’ health and safety was “of utmost importance”, it said, adding there were no reports of ill health.
One woman contacted the BBC to say she became ill after swimming on Wednesday.
Further water sampling is due to take place later and on Monday, the port added.
The woman, who wishes to remain anonymous, said she swam in the sea at Dover with her family on Wednesday and became unwell that evening.
“Both my husband and I have had diarrhoea since Wednesday night,” she said.
“The friends we were with have also been poorly. None of us thought anything of it until Dover port put out their tweet last night.”
‘Utter embarrassing shambles’
In an earlier statement, the port said that when “water standards were not 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.”
Results from further water testing are expected to be known on Tuesday.
Some local residents remain concerned however.
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”.