The supplier said demand had reached its highest levels for 25 years as a result of one of the driest starts to a year on record.
Thames Water said underground water levels had now fallen below average for the first time this year.
A Thames Water spokesperson told the Standard: “The recent heatwave and extreme heat has resulted in extremely high demand, some of our highest for over 25 years.
“Our teams have been working 24/7, during incredibly hot conditions, to maintain supplies to customers, ensuring the output of our supply systems are running at maximum capacity.
“At times the demand can outstrip our capacity to treat water and hence we have been promoting the need to use water wisely to ensure we can maintain supplies for all.”
The supplier began the first stage of its “drought plan” in May but warned the next phase would be to implement a temporary use ban, including on hosepipes.
“The timing will depend on the amount of water used by our customers, which determines the speed at which reservoir storage declines, and the amount of flow in the rivers, which determines how much water we can take to refill them.”
In order to try and manage levels, Thames Water has asked customers to limit their consumption.
Southern Water also announced a ban last week for customers in Hampshire and the Isle of Wight.
According to the Met Office, last month was the driest July since 1935, while the first seven months of the year were the second-warmest on record.
