Some local people could be forced to sell their homes
A full public inquiry should be held into proposals to build a reservoir in South Oxfordshire, Conservative MP Ed Vaizey has said.
In September, Thames Water was told to revise "substantial parts" of its plans to build the £1bn reservoir.
Speaking in the House of Commons, Mr Vaizey said people had been living with the possibility of the reservoir being built south of Abingdon for 20 years.
He said Thames Water had not convinced locals of the need for it.
"A few are opposed to the reservoir come what may, quite understandably," said Mr Vaizey, MP for Didcot.
"Some would welcome the reservoir as a leisure resource and possible nature reserve.
"The vast majority of my constituents say to me that if Thames Water can convince me that it is essential, then they will reluctantly accept it."
The reservoir, planned for farmland near Abingdon, would be the biggest built in the UK in 25 years and hold 150bn litres of water with the majority of the supply destined for London.
People living between the villages of East Hanney and Steventon would be most affected by the reservoir, with the company warning of compulsory purchase orders on some homes.
If the scheme is approved construction is likely to begin in 2011 and is due to last about eight years.