Thames Water has gone out to tender today with a massive £6.84 billion (inc VAT) contract for the construction of the proposed South East Strategic Reservoir Option (SESRO) in Oxfordshire - now formally renamed as the White Horse Reservoir.
The current estimated total value is estimated at Q2 2025 prices and therefore excludes any allowance for inflation over the life of the contract from this date.
The 150 billion litre reservoir represents the largest water infrastructure investment in a generation – the reservoir has been designated as a Nationally Significant Infrastructure Project.
A Construction Engineer and Reservoir Advisory Panel have already been appointed to oversee the design and construction, ensuring adherence to safety standards and regulatory requirements throughout the project lifecycle. The Construction Engineer appointment will continue through the entire construction phase, through first filling and the initial years of operation.
The Reservoir Advisory Panel will provide regular review of the design and construction activities and draw upon best practice from around the world. The reservoir embankment and associated structures will be designed to national and international standards and guidance documents.
The company is now seeking a Main Works Contractor to design, build, test and commission the reservoir which would be situated south-west of Abingdon. The proposed reservoir would supply water locally in Oxfordshire and Wiltshire, through to Hampshire and London.
The White Horse Reservoir is a proposed fully bunded, non-impounding raw water storage reservoir in the upper catchment of the River Thames. Water will be abstracted from the River Thames during periods when sufficient river water is available and conveyed to the reservoir via a pumping station and circa 4km of tunnels.
To deliver the reservoir, watercourse diversions and replacement floodplain storage are required.
A conveyance system will be implemented to transport water to and from the River Thames, and to the interface points with the Thames to Southern Transfer (T2ST) and Farmoor Transfer projects.
The reservoir includes passive design provisions for future integration with other strategic water resource and flood alleviation schemes.
Thames Water is leading the development of the new reservoir project, in partnership with Affinity Water and Southern Water.
The UK's biggest water company is responsible for delivering the development phase, developing the commercial model for the project and for running the procurement processes to appoint the project's Main Works Contractor (MWC) and later, the appointment of an Infrastructure Provider (IP).
The Main Works Contract comprises the award of a multi-supplier Framework Agreement and a single Call-Off Contract - the Call-Off Contract will be awarded immediately following the establishment of the multi-supplier Framework Agreement.
The Call-Off Contract involves the design, build, test and commission of a new reservoir with an operational capacity of 150 million cubic metres (Mm³) and water surface area of circa 6.7km2.
The Call-Off Contract will be based on the NEC4 Engineering and Construction Contract June 2017 using main option C, secondary options including clause X22 (early contractor involvement ECI)), and bespoke amendments,
The Call-Off Contract two-stage ECI approach comprises:
- Stage One - a design and construction planning stage
- Stage Two - a detailed design and build stage
At the end of Stage One, the Call-Off Contract will be novated to the IP from Thames Water. From the point of novation, the IP will become the Main Works Contract client and contract counterparty throughout Stage Two, administering the Call-Off Contract and making payments to the MWC accordingly.
Current estimated start and end contract dates are 11 August 2027 to 31 March 2043.
All applicants wishing to participate in the competition for a Main Works Contract for the White Horse Reservoir must demonstrate in their submissions that they meet the Conditions of Participation and all other requirements set out in the Procurement Specific Questionnaire Pack (PSQ Pack).
The PSQ Pack includes a summary of the proposed:
- Award criteria
- Call-Off Contract Scope, and
- Framework Agreement and Call-Off Contract terms and conditions
Thames Water is holding an online Applicant Briefing on 11 February 2026 regarding the Request to Participate stage and the Main Works Contract opportunity. The briefing will provide an overview of the requirements set out in the PSQ Pack. Applicants wishing to attend the briefing must register using the contact details in the PSQ Pack Part 1 (section 1.2.4), by 12 noon on 10 February 2026.
Registration requests will be subject to confirmation, following which confirmed attendees will be issued with joining instructions.
For appliants requesting access to the PSQ pack after the Applicant Briefing has taken place, a recording of the briefing will be available in Thames Water’s e-Tendering System once they have completed their registration.
Deadline to submit enquiries about the contract is 11:00am on 7 April 2026 and deadline for submission of requests to participate is 11:00am on 24 April 2026.
Click here to access the link which will take interested parties to the Applicant Registration Form. Completing the Applicant Registration Form will then, subject to confirmation, provide suppliers with access to the secure site containing the Procurement Specific Questionnaire Pack (PSQ Pack) and submission portal.
The estimated contract award decision date is currently 13 July 2027.
Thames Water is delivering the project under the Specified Infrastructure Project Regulations (SIPR), first established and used for the Thames Tideway Tunnel project.
Under this approach a new regulated entity, known as an IP will be established to finance and deliver the project, holding a Project Licence granted by Ofwat. Delivery under SIPR is subject to a notice to specify an infrastructure project, which is currently under development between Thames Water and Ofwat.
Following the approach taken on the Thames Tideway Tunnel project, Thames Water is separately procuring the Main Works Contract and the IP Contract.
Chris Weston, CEO, Thames Water said the reservoir represents one of the most significant investments in water infrastructure the country has seen in decades.
Chris Taylor-Dawson, Senior Director for Major Projects at Ofwat added:
“Kick-starting the procurement process to select the main contractor is a significant milestone in the development of this important project - critical for securing our future water supply in the South East of England.
“Building on the success of the Thames Tideway project, using a competitive third-party delivery approach will boost resilience in the sector and deliver better value for customers.
“Engaging contractors at an early stage in the development of reservoir will tap into specialist expertise and innovative solutions – all of which can help lower costs in the longer term.”
Interested organisations have until 24 April 2026 to submit a request to participate, after which shortlisted applicants will be invited to submit detailed tenders.