Thames Water has fired the starting gun on the “extensive” process for appointing a main contractor to design, build, test and commission its major new Oxfordshire reservoir in a deal worth £5.7bn.

The contractor for the White Horse Reservoir – renamed from the working title South East Strategic Reservoir Option (Sesro) – will need to have “experience and expertise of constructing a large scale asset” and be comfortable working in a “complex, highly regulated environment”, Thames Water head of procurement and supply chain Mohit Farmah told NCE.

The scheme is designed to store 150Mm3 and would cover roughly 6.7km2 near Abingdon. Thames Water proposes that water abstracted from the River Thames during periods of high flow would be pumped and conveyed via a new pumping station and about 4km of tunnels to the site.

Beyond water storage, the reservoir is being framed as “a place for people and a space for nature”, with the opportunity to sail and paddleboard alongside new wildlife ponds, wetland mosaics, woodlands and rich grasslands.

Tender details and contract structure

Thames Water intends to deliver the project’s development phase and run the procurement process for both the main works contract and a separate contract for an infrastructure provider, which would ultimately finance and manage the completed project.

Farmah told NCE that the procurement strategy has been two years in the making and has involved plenty of engagement with the market to ensure Thames Water releases “a set of terms that doesn’t deter the market”.

“We’ve tried to take a fair and balanced approach,” he continued. “We want to come across as an attractive client; we want to make sure that we have fair risk share, but we want to make sure that we’re protecting our customers from a value perspective.”

The procurement covers a multi-supplier framework agreement and a single call-off contract. The call-off is structured under an NEC4 engineering and construction form with an early contractor involvement (ECI) two-stage approach: Stage One focuses on design and construction planning, and Stage Two on detailed design and construction. Progression to Stage Two will depend on several “gate” conditions, including appointment of the infrastructure provider, an accepted Stage One design, confirmation of a target price and the grant of a Development Consent Order (DCO).

Farmah said that Thames Water has “taken the supply chain on the journey” to determining how it will run procurement, which has made transparent its expectations.

“Because of that, I think we’re in a strong position,” he said. “We’ve had approaches from some of the big Tier 1s, which is our target market. We’re quite confident we’ll have a good response.”

The deadline for initial expressions of interest is 24 April and the intention is to select “approximately four” applicants to invite to tender, according to Farmah. The tender process will start around August and last until spring 2027, with mobilisation on site anticipated for summer 2027.

Thames Water expects the contract period to run from about August 2027 to March 2043, with the reservoir targeted to be operational from 2040 – although filling could take years and some landscaping and visitor facilities work may continue until 2043.

It is looking for a contractor with a “track record” of constructing large scale assets, but also “the stuff that is less talked about”, according to Farmah.

“Experience of how health and safety is managed on site, for example; having the right policies in place from a sustainability perspective; stakeholder management,” he said.

The works will include earthworks to create embankments and borrow pits, tunnelling and pipelines, pumping and intake structures in the Thames, road and rail access including a temporary rail siding on the Great Western Main Line, utility diversions, environmental mitigation and habitat creation, and recreational infrastructure such as a visitor centre and angling piers.

Thames Water will be looking for a contractor with experience in these areas.

“At the tender phase we’ll be asking them how they intend to deliver this project specifically,” Farmah said. “There’s quite an extensive process in terms of selection and we’ve got the right SMEs and experts involved in the evaluation process.”

Farmah believes that the size of the project will probably attract applications from joint ventures (JVs). “Through the market engagement we asked our potential suppliers whether they would come alone or in a JV and 80% said they would come in a joint venture,” he said. “A lot are already established from projects such as HS2, Lower Thames Crossing and so forth.

“I’m quite confident we will get a single contractual interface but may have multiple organisations on the other end supporting us as a Tier 1.”

The water company is still working on its DCO application, which it is aiming to submit this autumn, with the contractor being officially appointed about half a year later.

Farmah refers to this as a “late ECI” agreement as the project will already be into its DCO examination. “The contractor will be with us to support us in the DCO, to help discharging some of the secondary consents, with the planning and logistics, derisking the delivery phase, bringing certainty in terms of price,” he said. “The contractor will be on board to reassure the Planning Inspectorate and the examiners from a construction perspective.”

Infrastructure Provider

The tender notice states that the £5.7bn contract value (£6.84bn with VAT) is at second-quarter 2025 prices and excludes inflation over the life of the contract.

Thames Water plans to use the Specified Infrastructure Project Regulations (Sipr) model for delivery – the same framework used for the Thames Tideway Tunnel – under which an infrastructure provider would hold a project licence granted by the industry regulator Ofwat. The infrastructure provider will fund the project and recoup its investment through a surcharge on Thames Water customer bills over the lifespan of the asset.

The water company opted to split the Sipr procurement to appoint a contractor sooner under an ECI model and an infrastructure provider in the following years.

“That allows us to build a credible proposition for investors, so when it comes to them to do their due diligence on the project they already have a ready-baked proposition to invest in and run with into delivery,” Farmah said. “Over the next couple of years we will start to form the basis of that infrastructure provider and the procurement for the finance will kick off in 2028.

“We hope to complete that process in 2029 so that when consent is achieved and we’ve got notice to proceed with the main works contractor then we’ve got the investment to go with it and it’s all go into delivery.”

Farmah is very confident of securing the finance for the project.

“This is a great news story for any investment firm; to be involved in a project that involves water,” he said. “Given the current agenda of investment firms – and even the main works contractors – in terms of their own ambitions regarding sustainability, I think’s a fantastic story to be a part of.”

Next steps

Thames Water is hosting an online applicant briefing for potential contractors on 11 February 2026 as part of the request-to-participate stage; registration closes on 10 February.

The White Horse Reservoir is one of nine new reservoirs proposed nationally to increase long-term resilience against drought and supply pressures linked to population growth and climate change.

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