The proposal arose in 2006 by
Thames Water. In 2007 the
Environment Agency opined that need for this was not proven. Further arguments were put but the near-term-demand case was rejected in 2011. In 2023, following a period of consultation, a revised version increased the proposal to . This would make Abingdon the second-largest reservoir in England by capacity, exceeded only by
Kielder Water in
Northumberland at , pushing
Rutland Water into third place at . Across the whole of the UK, only seven Scottish
lochs have
greater freshwater storage by volume. Additional storage capacity is expected to be needed by 2043 to cater for projected population growth in the
Thames Basin. In early 2025, Thames Water opened discussions with potential contractors about delivery of the reservoir, which would cost up to £2.2bn to build and would then supply water to 15m people across southeast England. Ground investigations were in progress to help inform an application for development consent in 2026. If the consent was granted, construction was expected to start in 2029 for opening in 2040.
Chancellor Rachel Reeves expressed her support for the scheme in a January 2025 speech as part of
"nine new reservoirs [...] to improve our water infrastructure and provide a foundation for growth." A judicial review of the proposed project started on 25 June 2025, but was dismissed in the High Court on 25 July 2025. In January 2026, Thames Water started the process to appoint a main works contractor for the £5.7bn project. == Reasons for the construction ==