Drilling for oil in the Surrey Hills will increase the risk of contamination to water supplies in the area, according to the area's water company.
Permission was granted in August for drilling near Leith Hill after a government inspector overturned Surrey County Council's original refusal.
Sutton and East Surrey Water director Lester Sonden said concerns remained over the future monitoring of the site.
Europa Oil and Gas said any drilling would be properly regulated.
The company was granted permission to drill an exploratory borehole and test for hydrocarbons at Bury Hill wood, near Coldharbour.
'Abandon water source'
Mr Sonden said it would mean drilling through water-bearing greensand rocks, running parallel to the M25, from which Sutton and East Surrey extract water.
If the aquifer became polluted the water source would have to be abandoned.
"The Environment Agency will require protection and monitoring of the site and that does reduce the risk," said Mr Sonden.
"But protection and monitoring are not forever.
"Europa Oil are not forever and neither are East Surrey Water but we are going to require those water sources at Dorking and Leatherhead for a long time to come."
Europa said it expected to drill the well in the winter of 2016/2017.
Chief executive Hugh Mackay said the Environment Agency would be very vigorous in applying the rules and monitoring Europa both before and after drilling.
"We feel the matter was considered properly by the planning inspector," he said.
"The public should feel confident that the industry is properly regulated.
The Environment Agency said in a statement: "Europa have proposed mitigation measures to prevent the groundwater at Bury Hill Wood from being contaminated.
"We will only issue an environmental permit if we are satisfied that the proposed works will manage the risks to groundwater."
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