Ofwat said the company “failed to operate, maintain and upgrade its wastewater assets adequately to ensure they could cope with the flows of sewage and wastewater coming to them.
It added that the company did not have in place adequate processes and oversight by senior management to ensure it was meeting legal requirements.
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It is the seventh firm to face action over sewage spills in recent times amid widespread anger over the issue.
The mood has not been helped by the imposition of inflation-busting hikes to household bills to improve vital infrastructure – pipe networks that have suffered from years of under-investment.
Ofwat, which is set to be replaced due to its own oversight record, said its proposed redress package would not come from customer bills.
It added that its wastewater investigations to date had brought in £300m from water firms to ensure improvements.
In the case of Welsh Water, the watchdog said £40.6m of company money would be spent “to address harm and reduce spills at specific overflows”.
It said sealing works would also be carried out on private parts of the sewer network to tackle groundwater infiltration, which is a significant contributor to frequently spilling overflows.
A total of £4.1m will be made available by WW to improve river water quality in “extremely sensitive catchments”, Ofwat added.
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Lynn Parker, senior director for enforcement at Ofwat, said: “Our investigation has found serious and unacceptable breaches in how Dŵr Cymru Welsh Water has operated and maintained its sewage works and networks, which has resulted in excessive spills from storm overflows to the environment.
“We now expect them to focus on putting things right so that customers can regain trust in their water company and the critical service they provide.
“We understand that the public wants to see transformative change. That is why we are prioritising this sector-wide investigation, which is holding companies, like Welsh Water, to account.”
Ofwat’s announcement coincided with the release of the company’s transformation plan which, it said, would strengthen accountability and performance.
Welsh Water chief executive, Roch Cheroux, said: “We know that in some areas we have not delivered the level of service our customers and communities expect, particularly on environmental performance.
“That is why we are investing at record levels to improve resilience, strengthen ageing infrastructure and deliver more reliable services, while keeping bills as affordable as possible.
“We are also listening carefully to customers and communities so that our future plans reflect the issues that matter most to them.”
