RAPID has written separately to the solution owners Thames Water, Southern Water and Affinity Water confirming the decision, together with identifying specific steps that the companies should take in preparing for gate four. The utilities submitted their gate three reports on 15 August 2025 for assessment.

Where the submission has not been assessed as ‘meeting expectations’ in the quality assessment, or progression concerns have been raised, RAPID has provided feedback on where it will seek remediation of the issues.

The regulators have categorised the remediation issues and steps into priority actions, actions and recommendations.

Priority actions are those that should have been completed at gate three and must now be addressed on a short timescale in order to make sure the solutions stay on track. They require urgent remediation in full. A total of 15 priority actions have been identified which should be delivered no later than the dates stated against each priority action.

A further 12 actions and 10 recommendations have been identified - actions are those that should be addressed in full by the gate four submission. The response to these actions will influence the assessment of the gate four submission.

Recommendations cover issues where additional information or clarification could improve the quality of future submissions.

The White Horse Reservoir solution is being developed by Thames Water, Southern Water and Affinity Water with the aim of delivering a new reservoir to store water abstracted during periods of high flow in the River Thames for use during periods of low river flow or high demand for water.

As part of the PR24 final determinations, Thames Water and Affinity Water proposed that Southern Water formally become a partner on White Horse Reservoir beginning 1 April 2025. This was agreed as part of the final decisions set out in the Major projects development and delivery appendix, allowing Southern Water development funding for the 2025 -30 period.

The partnership arrangements are now as follows:

White Horse Reservoir is expected to be financed and delivered through a competitive delivery model under the Specified Infrastructure Provider Regulations (SIPR).

Under SIPR designation, a third-party Infrastructure Provider would finance the project and assume responsibility for its delivery, rather than Thames Water. The Infrastructure Provider would be responsible for the majority of the project scope and for arranging the associated financing. Thames Water’s role would be limited to early preparatory activities, including the procurement of a Mains Works Contractor to develop the detailed design and undertake the majority of construction on behalf of the Infrastructure Provider.

Once the reservoir is operational, the high-level approach is that Thames Water will retain the day-to-day operational responsibility necessary to meet compliance obligations and undertake network integration activities.

The Infrastructure Provider will own and have responsibility for the major civil assets, including the embankments, and hold maintenance responsibilities for them. The detailed arrangements are still under discussion, however the full responsibility for delivery of the White Horse Reservoir will not fall to Thames Water.

Thames Water, Affinity Water and Southern Waterhad proposed that following Ofwat’s PR24 final determination, the gate four allowance should be updated from the originally allocated £99.47 million to £298.19 million (2022/23 price base) and requested that RAPID incorporates the updated allowance in its final decision.

Ofwat agreed with the evidence set out and has approved an additional £163.92 million for the project's gate four activities. Alongside this, £34.80 million has been reallocated from post gate four funding to enable early activities to be carried out.

The increased funding covers additional activities to de-risk the planning process including additional surveys, the earlier acquisition of land and increased scope of the Early Contractor Involvement (ECI) procurement process. Ofwat agreed with the evidence set out by the companies and the need to de-risk the project early.

The RAPID decision document states:

“We consider the evidence provided by Thames Water, Southern Water and Affinity Water regarding the programme and planning, risks and issues and the procurement and planning route strategy for this solution to be good overall, however it was felt that more detail could be provided in the project programme to identify key milestones.”

Click here to download RAPID’s final decision document