The claim was made by campaign group GARD (Group Against Reservoir Development), and backed by local MPs.
According to GARD, costs for the White Horse Reservoir have tripled – within months of preliminary government approval projected costs rose from £2.3bn to £5.5bn–£7.5bn.
READ MORE: Campaigners slam tests for giant new reservoir
Water company bosses, however, suggested that the new reservoir was the “best value option for securing water supply for 15 million people across the South East”.
Leonie Dubois, head of engagement, land and consents for Thames Water, said: “White Horse Reservoir is the biggest water infrastructure investment in a generation, and would provide water security for millions of people across the South East, which is why work to enable its delivery is well under way.
“Last summer we shared a revised cost estimate with our regulators (£5.5bn to £7.5bn), reflecting the detailed design work undertaken to date.
“We remain confident the reservoir represents the best value option for securing water supply for 15 million people across the South East, including Thames Water, Affinity Water and Southern Water customers.
“We continue to engage with the local community, and at our recent statutory consultation showcased the latest reservoir design and embankment plans alongside a detailed construction timeline.
“As is common with projects of this size, scale and significance, we will continue to refine the design as we move through the consenting phase. Feedback gathered during our consultation will also shape project plans ahead of our Development Consent Order application to Government.”
Ms Dubois set out the water company’s case after the latest attack on the plans from campaigners, who said so far a total of £543m has been spent on the project – which would be as big as Gatwick Airport.
Leonie Dubois, head of engagement for Thames Water (Image: Thames Water)
Supported by local MPs, Olly Glover, Charlie Maynard, and Layla Moran, Group Against Reservoir Development (GARD) has written to the water regulator Ofwat, suggesting the project still has no final design and no published construction time plan.
Derek Stork, GARD chairman, said: “Large infrastructure projects normally reach the planning stage only after detailed engineering design and project delivery scheduling are substantially developed.
“For the White Horse Reservoir, these core elements are missing, making it impossible to produce a credible construction methodology, realistic timetable, or a defensible forecast of final costs.
“This is why we’re requesting that the revised scheme receives a proper assessment against the viable alternatives.”
The water company said it has been “clear on a delivery date of 2040 for water to be available for use from the reservoir”.
It added that the South East is facing “serious water stress” due to climate change, population growth and environmental pressures.
Diggers at a test site near Steventon (Image: Thames Water)
The project would increase storage capacity, secure long-term water supplies, help increase drought resilience, and leave a lasting legacy for future generations, according to Thames Water.
The reservoir south west of Abingdon would produce up to 270 million litres of water per day.
Last year, embankment tests were carried out by Thames Water at a site near Steventon.
The reservoir is expected to have embankments up to 25m high.
