Around 70,000 people in north Wales are being told to keep boiling their water after bacteria were found at a treatment works.
The advice was first issued on Tuesday in parts of Flintshire and Denbighshire and later extended to an area of Conwy.
Welsh Water said an analysis of samples taken across the community supplied with water from the treatment works showed "no major cause for concern".
But customers should keep on boiling tap water as a precaution, it said.
There is no evidence anyone has become ill as a result of drinking the water from the works on Denbigh Moors the company said.
The incident is not being treated as a public health incident.
The bacteria- coliform - were discovered on Tuesday night and an operation put into place immediately to inform people living in the areas affected to boil their water.
We apologise for the inconvenience caused to our customers and assure them that we are doing all we can to resolve this issue as quickly as possible Peter Perry, Welsh Water
Welsh Water is providing regular updates on the situation.
In the latest, issued on Thursday afternoon, operations director Peter Perry said: "We have instigated a major water sampling programme across the community supplied with water from the treatment works, and the analysis results to date show no major cause for concern.
"However we still do not have enough information to enable us to lift the Boil Water Notice, which still remains in place, as a precaution, as agreed with local health authorities.
"We are also continuing our efforts to make sure everyone affected by the Boil Water Notice is aware of it by loud-hailing, poster distribution and regular media updates."
He said vans had been out on Wednesday loud-hailing the boil water message across the area affected, concentrating firstly in the most densely populated towns and villages, until 2030 BST.
This would continue until the same time on Thursday evening, he said.
Five hundred advisory posters had been delivered to supermarkets and libraries and information packs had also been delivered to schools, so that pupils could take the message home with them, he said.
Mr Perry said customers who are unsure as to whether the boil notice affects them they can call our 24 hour helpline or consult Welsh Water's website.
The National Public Health Service for Wales (NPHSW) is maintaining close surveillance of the population affected to monitor for any illness.
Future contamination
Mr Perry continued: "We know that this situation is inconvenient to our customers and compensation is something that we will be considering, however, at this stage of the operation we are concentrating on making sure that everyone affected knows about the boil water message, and trying to locate the source of the bacterial problem."
He apologised for the inconvenience caused and wanted to assure customers the company was doing all it could to resolve the issue "as quickly as possible, so that the boil water notice can be removed".
On Wednesday Delyn AM Sandy Mewies asked an urgent question in the Welsh assembly, calling for "assurances that action will be taken to prevent future contamination".
This is the updated list issued by Welsh Water of towns and villages affected by the advice:
