A DRINKING water alert has been issued for the Lower Clarence.

Clarence Valley Council (CVC) issued the warning after test results revealed unacceptable levels of Escherichia coli (E coli) in the water supply.

Frank Vaarwerk, an operations engineer with CVC, said although council could not be 100 per cent certain, the problem was most likely caused when pump rates were increased to lift chlorine levels in the Maclean reservoir.