Southern Water was fined after pleading guilty to one breach of the Environmental Permitting (England and Wales) Regulations, relating to a 2019 pollution incident in Little Bull Lane, Hampshire.
Richard Manning, General Counsel and Company Secretary said:
"We are very sorry that this unacceptable historical incident in 2019 led to environmental damage.
"As soon as we became aware of this event, we took action to reduce its impact on the local area, and have since co-operated fully with the Environment Agency’s investigation, pleading guilty at the first opportunity.
"In acknowledgement of our role, we have already compensated the YMCA and set up a £140,000 grant scheme with the Groundwork South Trust to aid habitat improvement.
"Learning from this incident also led to a comprehensive review of our more than 3,000 unmanned pumping stations to ensure those at highest risk were fit for purpose, leading to a rolling programme of improvements to equipment and monitoring technology which has already cost more than £13 million.
We have learnt from this incident and welcome the judge's comments that the company has made good progress in its turnaround plan since 2019 and more recently.
Five years on from this event we have a new leadership team, new shareholders and we are spending £3bn - the equivalent to £1,500 per household - between 2020 and 2025 to improve performance."