Affinity Water has warned that a water-supply fault may affect numerous properties in the NW7, NW9, HA8 and HA9 postcode areas on November 9.
The affected locations include Mill Hill, Edgware, Colindale, Kingsbury, Queensbury, Burnt Oak, Canons Park and Wembley, leaving people across north-west London and Edgware without access to water in some cases.
The cause of the problem remains unknown and is being investigated.
Affinity Water confirmed that technicians are travelling to the affected area to assess the fault.
Households that still have access to water have been advised to place some in a kettle or fridge for drinking, in case the supply is switched off while repairs are carried out.
Washing machines, dishwashers and other electrical appliances that require water should also not be used.
Affinity Water apologised for the disruption and said work is under way to restore normal water service as quickly as possible.