Water firm United Utilities has pleaded guilty to supplying water that was unfit for human consumption to hundreds of thousands of customers.
The firm is being prosecuted by the Drinking Water Inspectorate after about 700,000 people in Lancashire had to boil their water in the summer of 2015 when microbial parasite cryptosporidium was found in the Franklaw water treatment works in Preston.
Lisa Roberts QC, appearing on behalf of United Utilities at Preston Magistrates' Court on Wednesday, entered the guilty plea to the charge of supplying water unfit for human consumption between July 30 2015 and August 18 2015.
District Judge Jane Goodwin agreed to formally withdraw charges of failing to disinfect water that was supplied for domestic purposes and failing to design and continuously operate an adequate treatment process before supplying water for domestic purposes.
Richard Banwell, counsel for the Drinking Water Inspectorate, asked for the case to be sent to Crown court.
He said: "It is likely there will be a substantial fine in this case."
The case was adjourned to August 21 when sentencing will take place at Preston Crown Court.