South East Water has applied for a drought order that would allow it to take more water from the River Ouse at Barcombe, near Lewes, in East Sussex.
Ardingly Reservoir is less than half full of water
It would then use extra river supplies for six months to preserve water levels at Ardingly Reservoir, in West Sussex after months of below-average rainfall.
The reservoir supplies customers in large parts of East and West Sussex.
Water resources manager James Grinnell said it was necessary to act to safeguard supplies in the long term.
"While the recent rainfall has been welcome and certainly kept peak demand for water at more manageable levels, it has not been substantial enough to make a significant impact on the low levels in our underground sources and reservoirs.
"Without substantial rainfall this autumn and winter, there could be the possibility of more serious water shortages next summer," he said.
South East Water also urged its customers in Sussex and West Kent to "keep up their effort" and not waste water over the coming months.
It introduced a hosepipe ban in July which were the first restrictions on water use since 1995.
Ardingly Reservoir is currently 43% full, while Arlington Reservoir is 47% full.
Mr Grinnell reminded customers that there was still a drought and the hosepipe ban remained in force.
"After months of low rainfall the levels in our reservoirs, underground sources and rivers are much lower than they should be for the time of year.
"We now need above average rainfall during the autumn and winter months to refill our resources or we could be facing more serious water shortages next summer," he warned.