Up to 3.6 million customers in south-east England have been told to save water, because of a lack of rainfall.
Affinity Water, which supplies parts of the home counties, north London and Kent, says many rivers in the region are low on water.
It said that since July last year, the region had received just more than half of its average rainfall.
Farmers across the country have also been affected by low moisture levels.
Affinity has asked its customers to save water to minimise the possibility of restrictions - such as hosepipe bans - later in the summer.
Among the ideas being suggested are:
- Stopping the use of sprinklers in the garden
- Taking a four-minute shower instead of a bath
- Turning off the tap while brushing teeth
- Only running dishwashers and washing machines with a full load
Hosepipe bans have not been seen in the UK since 2012, when seven water companies introduced restrictions.
Farmers in the south-east and eastern England say they have had just 10% of the expected rainfall since the beginning of March.
Crops such as spring barley and sugar beet have been hit, because a lack of moisture in the ground has hindered germination.
The National Farmers Union said there was also concern about some fruit and vegetable farms.
The low rainfall has affected some livestock farmers as far north as the Midlands, where there has been a shortage of grass to feed beef and dairy herds.
A spokesman said it was too early to say whether agricultural production would be down, as that would depend on the weather over the next few weeks.
However, prices are unlikely to be affected, as they are determined by global markets.
Concern appears to be restricted to areas of the South East. Thames Water, which supplies nine million households in the region, has no problems at the moment.
It draws its water from rivers and reservoirs, while Affinity relies on ground water.
But Water UK, which represents the industry, said everyone should consider using water carefully.
"Following a dry winter, some rivers, groundwaters and reservoirs are lower than normal for the time of year," a spokesperson said.
"We always advise that everyone use water wisely - especially during a period of dry weather - and to follow the advice of their water company should water saving measures be required."
Customers of Affinity Water are being offered some free devices, such as economical shower heads, devices to save water in toilets, and shower timers for children.