Dwr Cymru Welsh Water is facing charges of supplying water unfit for human consumption after a stomach bug left hundreds ill in north Wales.
Hundreds of people were affected by the stomach illness
The bug cryptosporidium affected more than 230 who drank water from a Snowdonia reservoir in November 2005.
The Drinking Water Inspectorate is bringing the prosecution against the company, which faces five charges.
The company - to appear before Caernarfon magistrates in July - said it was "disappointed" by the decision.
About 70,000 homes in parts of Gwynedd and Anglesey whose water was supplied from the Llyn Cwellyn reservoir at Rhyd-ddu, were told to boil their drinking water for two months after the outbreak.
A total of 231 people fell ill with the bug which causes vomiting and diarrhoea.
Health experts said at the time that the likely source was the Llyn Cwellyn reservoir, although tests at the lake always proved negative.
In a statement Welsh Water said: "Protecting public health by providing a safe and reliable water supply has always been and will always be out first priority.
"It is crucially important that our customers continue to trust their tap water."
The company also said that an independent report into the incident by the Outbreak Control Team - which includes the Environment Agency, councils, Welsh Water and public health bodies - concluded its Cwellyn water treatment works was the likely source of the cryptosporidiosis.
The firm said that report concluded "it is difficult to see how the outbreak could have been prevented, or its impact reduced, given what was known before the outbreak started and what became apparent as the outbreak developed".
Since the outbreak, the water company has installed new ultra-violet treatment to kill the bug.
Welsh Water added: "Our investigations at the time confirmed that the Cwellyn water treatment works was operating normally, and all testing for water quality showed no breach of regulatory standards in force at the time.
"It is only since the outbreak that the Drinking Water Inspectorate has reviewed and issued new guidance to the industry and has begun a consultation process on changes to the relevant water quality regulations."