Thames Water said the measure would come into effect on 22 July for households across Gloucestershire, Oxfordshire, Berkshire and Wiltshire who were supplied by Farmoor Reservoir near Oxford.
Announcing the ban, Thames Water said a lack of rain and increasing demand had stretched water supplies.
The temporary restriction will affect people across areas with postcodes beginning with OX, GL, SN, RG4, RG8 and RG9.