Thames Water is launching a major consultation for a giant new reservoir near Abingdon to secure water supply for 15 million customers.
The statutory public consultation runs for 11 weeks until January 13.
It provides the latest opportunity for people to share their views, which can be included in the application for planning consent, which the company will submit to the Government in Autumn 2026.
Major opposition to the new reservoir has already been declared, with Vale of White Horse District Council agreeing to call on the Government to rethink its decision to back the plans.
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The South East Strategic Reservoir Option (SESRO) would be situated near Abingdon, providing up to 271 mega litres of water per day during drought, the equivalent to around three million baths. The new reservoir will be the size of Gatwick Airport.
The consultation represents a critical step for the project with responses helping to shape the Development Consent Order application Thames Water will submit to the Secretary of State next year.
Should permission be granted, water would be available for use from 2040.
Leonie Dubois, head of engagement at Thames Water (Image: Thames Water)
Leonie Dubois, head of engagement, land and consents at Thames Water, said: "The need for this reservoir has never been more urgent. Year after year, we're witnessing more frequent droughts and prolonged dry spells across the UK, and this summer was no exception.
"Our proposed reservoir is one of the most vital infrastructure projects in the country. It will safeguard water supply for millions of people across the south east, including local communities, protecting our precious rivers and groundwater sources in the process.
"The statutory consultation is the most significant milestone in our reservoir journey to date. Public feedback will shape our application for planning consent, which we will submit to Government next year.
"I'd encourage everyone to look at our latest plans, attend our events and speak to the team to see what the reservoir means for them."
In January, Chancellor Rachel Reeves backed plans for the new reservoir, and GARD's (Group Against Reservoir Development) sister company Saferwaters, and countryside charity CPRE Oxfordshire, launched a bid for a judicial review.
Then Defra (Department for Environment, Food and Rural Affairs) was called to the judicial review in July, but it was not successful.
Campaigners, however, vowed to continue their fight against the reservoir, and GARD is continuing to challenge Thames Water on the issue of the reservoir's costs.
Sarah James, leader of the Vale council's Green group (Image: Sarah James)
Sarah James, leader of the Green group who represents Hendreds, also questioned the rising cost estimates, adding: "Thames Water have got a £2.2bn reservoir through the process so far, and then just seem able to whack the price up as high as they like with no further questions.
"People are angry and upset about the lack of accountability."
Earlier this year, embankment tests for the new reservoir were carried out near Steventon.
Compacted clay was formed into 3m (10ft) walls to test ways of containing up to 150 billion litres of water. The reservoir is expected to have embankments up to 25m (80ft) high.
The estimated cost of building the reservoir has risen significantly from £2.2bn to between £5.5bn and £7.5bn, and the estimated operating cost is £1.9bn for the first 65 years.
The reservoir will serve Thames Water, Affinity Water and Southern Water customers.
An artist's impression of Abingdon Reservoir (Image: Thames Water)
If the reservoir goes ahead, it is expected that customers will pick up the bill.
According to the Guardian, about half the costs are due to be recovered from Thames Water's 16 million customers across London and the South East, with Affinity Water and Southern Water customers sharing the rest.
Thames Water customers already face a 35 per cent increase in bills over the next five years.
The water company staged a consultation exercise last year, but it was not compulsory.
As part of the consultation, there will be seven in-person and two online events throughout November.
People can meet Thames Water's team of experts to find out more and ask questions.