Thames Water has been granted emergency powers to see off a potential drought.
Thames Water say they need four weeks of heavy rainfall
After one of the driest periods on record, levels at Farmoor reservoir in Oxfordshire, the largest single area of open water in the county, are less than half their usual level.
The company say they need they need the right kind of rain to replenish stocks, as torrential rain will overflow rivers and cause floods.
The Environment Agency has now given Thames Water a drought permit to suck out extra water from rivers and bore holes.
The company says the situation in Berkshire is not so serious, but they will review it in the new year.
Thames Water says some areas of the Thames Valley have received as little as 30% of their average rainfall for the summer.
It says another four weeks of persistent rain are needed for river flows and groundwater levels to begin a recovery, and rainfall between now and March will need to be well above average for the time of year.