The warning was issued after water readings revealed abnormal levels of intestinal bacterial enterococcus, for at least the second time in the past few days.

The Coast Guard (Rannavalve) has hoisted a purple warning flag to indicate contaminated water and, while bathing is not forbidden, authorities recommend giving it a miss.

The beach had briefly been declared safe to bathe.

Children, the elderly and those with compromised immune systems are particularly susceptible to enterococcus contamination.

Manuela Pihlap of the North Tallinn district of the capital's city government says that while water samples from Kopli Bay taken on July 15 prove, upon analysis, to be acceptable for bathing, those taken four days later did not.

The Health Board (Terviseamet) has taken more samples since then, while Tallinn city government says it has also installed signage counseling against swimming off Stroomi beach.

An environmental impact assessment should help to identify the origin of the bacteria, authorities say, which may consist of more than one source.

Kopli Bay is enclosed and shallow and encompassed by a fairly densely-populated residential area, with streams and sewage outlets, industry as well as an equestrian center nearby.

A recent oil slick in the bay took several hours to clear up.

Pikakari beach, on the other side of the Kopli peninsula, maintains good water quality, the city government says.

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