The reservoir would force some people to leave their homes
Plans to build the UK's largest reservoir for 25 years at a cost of £1bn have been scaled down.
Thames Water had originally proposed a 10 sq km (3.9 sq mile) reservoir, which would hold 150bn litres, between East Hanney and Steventon in Oxfordshire.
However, the company's has said it now intends to build a facility which can hold 100bn litres.
Thames Water said the economic downturn had contributed to its forecast for the demand for water being reduced.
The company, which has warned of the need for compulsory purchase orders on some homes, also said the start date for the project had changed.
Construction was due to begin on the site near Abingdon in 2011 and was scheduled to take eight years, but the project is now scheduled to be completed by 2026 at the earliest.
By 2035, Thames Water is expecting people to use 135 litres of water each a day, compared with a previous forecast of 157 litres.
Thames Water chief executive David Owens said: "The economic downturn has had a significant impact on our water demand forecasts.
"As a result, we are now predicting a slower increase in population and household numbers."
Mr Owens added: "Additional water resources will be required by 2020 and, after carefully re-examining all the options, we still believe there is a strong case for a major new reservoir in Oxfordshire.
"However, in view of the scale of the outstanding uncertainties it is appropriate to promote some smaller schemes first."
A statement released by Gard (Group Against Reservoir Development) said it was encouraged by the announcement.
"We need to examine the detail of their figures and the revised document, but are encouraged that the reservoir proposal has been put back by five years, and its size reduced.
"We are still clear we can demonstrate that a reservoir is not needed."
Thames Water's revised plan has been sent to Environment Secretary Hilary Benn for approval.