Water quality results from Balbriggan beach in north county Dublin show gross contamination of the bathing water after raw sewage was pumped into the sea earlier this week.
Fingal County Council said the levels of E coli found in the samples were way beyond permitted levels.
The beach will remain closed until the results of further tests are known.
Raw sewage was discharged into the Irish Sea for up to 26 hours after a power cut caused a malfunction to a pumping station.
A member of the public raised the alarm on Tuesday afternoon when the waste water was washed back in with the tide.
Fingal County Council said test results show E coli at 10,462 per 100ml and enterococci at 1,310 per 100ml.
Test results from the neighbouring Blue Flag Skerries beach have returned clear.
