Irish Water said last night that water quality samples have detected cryptosporidium in the Westport Public Water Supply.
This means the boil water notice issued to thousands of Westport homes and businesses will remain in place.
Cryptosporidium is a microscopic parasite that can cause the potentially serious diarrhoeal illness, cryptosporidiosis.
Westport householders were put on the boil water notice on May 6 following eight confirmed cases of cryptosporidiosis in the Westport area.
No cryptosporidium was initially found in water quality samples but Irish Water confirmed last night the presense of the parasite in the public water supply.
"Investigations are continuing throughout the water network to identify any issues and the boil water notice on Westport Public Water Supply remains in place," reads the Irish Water statement released last night.
"An Outbreak Control Team chaired by the HSE, has been set up to manage this incident with senior representatives from Irish Water, the HSE and Mayo County Council.
"Irish Water will continue to provide regular updates through local media outlets on progress being made to allow the boil water notice in Westport to be lifted.
"The lifting of the boil water notice is subject to the HSE and EPA being satisfied that there is no risk to customers from the water supply and that all relevant procedures have been followed.
"Irish Water and Mayo County Council apologises to all affected customers for the inconvenience and disruption the imposition of this boil water notice has caused."