The company which supplies water to Catterick Garrison says it has traced the source of a sickness bug.
Thousands of army staff and civilians were told on Tuesday to boil their water after traces of the cryptosporidium parasite were found.
Water company C2C Services said a problem with one of the four main filters at the treatment plant was thought to be the cause.
The filter has been isolated and further tests are being carried out.
The company said the tests would confirm whether the fault was in the filter.
Schools closed
It said: "When it is confirmed we will be carrying out flushing of the water in some areas and undertaking further water quality sampling tests.
"We anticipate the boil notice will have to remain in place for a further three days.
"We are not aware of any reports of illness."
All personnel within the military area and 2,000 commercial and residential properties outside are affected.
Four schools in the Catterick area closed on Tuesday because of concerns about the water supply.
Everyone within the affected area has been advised to boil all water used for drinking, cleaning teeth and food preparation.
Residents in the area who are supplied by Yorkshire Water are not affected and do not have to boil their water.
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