Ofwat is investigating Southern Water’s performance of its wastewater treatment sites and reporting of compliance information.
It will look into whether the water company has breached its statutory duties and licence obligations.
Ofwat stated: “We will conclude the investigation as quickly as we can to establish whether or not there has been any contravention and whether we need to take any enforcement action. We will not be providing any further comment on the investigation at this time.”
Southern Water said it is fully co-operating and assisting Ofwat with its inquiries.
A spokesperson added: “We are committed to working with Ofwat and all our regulators to ensure that we continue to improve our services.
“As these investigations are ongoing, we are limited on what we can say at this time but we will provide further updates on the situation – and the work that we are doing – once we are able to do so.”