Thames Water has opened a £5.7 billion tender to appoint a Main Works Contractor for the proposed White Horse Reservoir in Oxfordshire, a 150 billion-litre scheme described as the largest investment in UK water infrastructure in a generation.

The reservoir is one of nine planned across the country to strengthen national water security, support the government's ambition for 1.5 million new homes, and bolster long-term resilience to climate change. The project, serving customers of Thames Water, Southern Water and Affinity Water, is intended to help secure supplies for up to 15 million people across the South East.

Located south-west of Abingdon, the reservoir would feed Oxfordshire and Wiltshire through to Hampshire and London. Construction is expected to generate around 1,800 jobs and stimulate economic growth, with Thames Water research indicating 73% of people would visit the site, positioning it as a new leisure and tourism destination.

Chris Weston, CEO, Thames Water, said: "The reservoir represents one of the most significant investments in water infrastructure the country has seen in decades.

"Once built it would be one of the largest in the country, providing water to millions, supporting new homes and development, and creating an exciting new destination where people can unwind and connect with nature.

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"Careers will be made and built through the reservoir's construction with thousands of jobs created, supporting families and communities across the South East."

Ben Ring, Major Projects Director, Southern Water, said: "White Horse Reservoir is an essential part of our plan to keep taps and rivers flowing in Hampshire.

"Combined with our Thames to Southern Transfer pipeline, it would supply our customers with up to 120 million litres of water a day during a drought, meaning the same amount can be left in Hampshire's rare and sensitive chalk stream rivers – the Test and the Itchen."

Stephen Plumb, Asset Strategy and Capital Delivery Director, Affinity Water, said: "As an industry it is crucial that we are planning for resilient water supplies in the future.

"Along with reducing leakage and helping customers to reduce their demand for water we are working together, across company boundaries, to ensure there are a range of schemes to deliver and support water resources in the UK."

Chris Taylor-Dawson, Senior Director for Major Projects at Ofwat, said: "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."

Reflecting local heritage, the scheme has been formally named White Horse Reservoir, replacing the working title South East Strategic Reservoir Option (SESRO). Beyond water storage, plans include a visitor centre and café, recreational lakes for sailing and paddleboarding, dedicated angling piers, education and adventure areas, and new habitats such as wildlife ponds, wetland mosaics, woodlands and species-rich grasslands.

The build is expected to create jobs across a range of disciplines, including ecologists, geologists, archaeologists, engineers, electricians, construction managers, park rangers and sailing instructors. Thames Water is engaging with schools, colleges and training providers to help develop a reservoir-ready workforce.