The apology comes after water supplies went down twice in Bognor Regis earlier this week: once this morning and once on Tuesday morning.
"Whilst we responded very quickly and rectified the matter so any disruption was for a short period of time we, as a business, pride ourselves on the service we provide," said Portsmouth Water CEO Bob Taylor.
"The fact this has happened twice in a week in the same area is clearly not satisfactory. We have now identified the issue and I can assure our customers that we are looking at this as a priority matter in order to prevent any repeat.
"Once again, I can only apologies for any inconvenience caused."