Low water supplies have led to a hosepipe ban being introduced in parts of Sussex, Southern Water has said.
The ban follows a dry winter
It follows repeated warnings from water firms in the region that restrictions are set to follow a dry winter.
Problems are worse in the South East because of the dense population, said Water UK, which represents suppliers.
Environmentalists want to cut people's water use and prevent house-building on flood plains, which stops rain reaching the aquifers that hold water.
The building of tens of thousands of homes in the South East is currently being allocated to district council areas.
Plans include building in the Gatwick, Ashford and Thames Gateway areas, in Surrey and along the Sussex coast.
Mark Shepherd, from independent environmental advisory company ADAS, said: "People have got to realise that we are a water-poor country.
"Out of all the European countries, we are near the bottom of the league table for the amount of water we have per head of population."
But Water UK said the other parts of Europe, including France, Spain and Portugal had a similar situation.
A spokesman said the South East of England had only had 60% of its normal rainfall between November and May.
Water-saving measures brought into force now were likely to be pre-emptive with supplies expected to be very low later in the summer, the spokesman said.
Climate change had also led to predictions that the UK would see more flooding and more drought in future years, because concentrated downpours did not soak into the ground well, he added.
Water UK has also said this year that problems with leaks in pipe networks, notably in London, would add to difficulties.
Consumer group WaterVoice said it was "not surprising" that drought measures were being brought into force given the lack of rain.
In April, Sutton and East Surrey Water banned the use of garden sprinklers and unattended hosepipes because of the continued low rainfall.