Up to 3.6 million customers in the south-east have been told to save water because of a lack of rainfall after one of the driest Aprils on record.
Affinity Water, which supplies water to homes in both north London and Kent, says many rivers in the provider’s counties are running low.
It has asked customers to save water to minimise the possibility of restrictions like the hosepipe ban later in the summer.
Among suggestions for how to cut back are taking four-minute showers instead of a bath, stopping the use of sprinklers in the garden and only running dishwashers and washing machines with a full load.
The UK has not seen a hosepipe ban since 2012, when seven water companies introduced restrictions.
But the Environment Agency has downplayed concerns of drought.
The agency said it was monitoring the situation following a period of "dry weather", but stressed that the UK was not experiencing critically low supplies.
In total, 34.7mm of rainfall was recorded last month, which is just under half the expected amount for the month as a whole (48%).
An Environment Agency spokeswoman said: "Following a dry winter, some rivers, groundwaters and reservoirs are lower than normal for the time of year.
"We always advise that everyone uses water wisely - especially during a period of dry weather - and to follow the advice of their water company should water saving measures be required.
"The Environment Agency, water companies, businesses and farmers are working together to minimise any potential impacts to people and the environment should the dry weather continue."
Met Office statistics show last month was the 10th driest April since records began in 1910. It follows the driest six-month period from October to March since 1995-1996.
The figures come amid concerns that low water supplies in parts of the UK could affect crops in the coming months.
Affinity Water is the only supplier to issue advice about conserving water ahead of the summer.
Concern appears to be restricted to areas of the South East. Thames Water, which supplies nine million households in the region, has reported no problems at the moment.
It draws its water from rivers and reservoirs, while Affinity relies on ground water.