A public inquiry has begun in Oxford into plans to build a £1bn reservoir.
Thames Water (TW) wants to build the facility, which would hold 100 million tonnes of water, on land near Abingdon.
A spokesman said water supplies in the area were already stretched and the reservoir would help ensure demand is met over the next 25 years.
Campaigners the Group Against Reservoir Development (GARD) claim the plan is not a "cost effective and environmentally sustainable" option.
In a statement, it said: "GARD's objective is to persuade the Government inspector chairing the inquiry to conclude that TW's Abingdon reservoir is not needed."
The Wantage & Didcot MP Ed Vaizey has campaigned for a public inquiry, which he hopes will allow all the arguments to be heard.
He added: "I have always maintained that Thames Water must be held to account and its plans scrutinised.
"There are perfectly viable alternatives to a reservoir, as well as clear ways to reduce water consumption so that a reservoir is not necessary."
TW identified the South Oxfordshire site more than five years ago and the inquiry follows a public consultation in 2007.
Martin Baggs, of TW, said the company supplies water to 8.5 million people.
"We take this responsibility extremely seriously. After consulting widely, our customers have told us they do not want us to take any risks with the security of their water supply," he added.