The recent flooding means a near-overwhelmed water treatment works off the A22 near Whyteleafe may have to be shut.
The works in Kenley supplies water to more than 46,000 properties.
But Richard Rap, operations manager (production) for Sutton and East Surrey Water said: "Its touch and go at the moment with each new downpour adding to the possibility of the site being overwhelmed with floodwater.
"We have teams working 24/7 and they will continue to do so until the danger has passed.
"While we hope for the best we are prepared for the worst."
Sandbags have been placed around key operating buildings and plant.
The company says that if it does have to shut down water treatment, it plans to supply its customers by re-routing additional supplies from other treatment works.
*Firefighters were called to a flood at the water treatment plant on Thursday evening.
About 20 firefighters used a specialist pump to ensure the water did not become contaminated.
