Update: A boil water notice affecting more than 615,000 people across Dublin, Kildare and Meath remains in place tonight.
Irish Water has said it could not lift the ban as it cannot guarantee water coming from the old treatment plant in Leixlip is safe to drink.
The issue was initially caused by a spell of heavy rain, with the utility saying it cannot say what effect tonight's Status Yellow rainfall warning will have on the plant.
Irish Water said the heavy rains led to “very cloudy” water entering the Leixlip facility. It said it has two treatment plants at the facility and while, the newer one was able to safely manage the influx of cloudy water, the older plant was not.
Met Éireann said tonight's Status Yellow rainfall warning for Dublin, Kildare, Louth, Wicklow and Meath will bring spells of heavy rains at times with “some spot flooding possible”.
In a statement, Irish Water said it is “not possible to say conclusively at this time” what effect the fresh rains will have on the water quality at the plant.
It said it will monitor the situation closely in the coming hours.
Irish Water said it is working with HSE and EPA officials to resolve the issue as quickly as possible.
615,000 people face a third day of boiling their water today in Dublin, Kildare and Meath.
Testing at the Leixlip treatment plant is continuing, after heavy weekend rain affected water quality and triggered a major boil water notice.
The boil water notice will not end until tomorrow at the very earliest.
Three test results proving acceptable quality levels must come back before the ban can be lifted.
The final sample is due later this evening, when a decision will then be taken by health authorities on the boil water notice.
The affected householders were impacted by a separate three day ban just two weeks ago.
Now Irish Water is warning of further boil water notices for people served by the Leixlip plant, until it is upgraded.
That is due to be done next summer.