The water supply is being restored after the fault was found in a water tower.
Affinity Water said the water supply in Barnet is reduced and even completely lacking after neighbours alerted them this morning.
After engineers went out to a reservoir and found no bursts, they identified a problem with equipment at the water tower in Arkley, which has since been fixed.
Areas in the EN5, EN4, N20, N11 & N12 woke up to a decreased water supply, with some homes without water at all.
A statement on the Affinity Water website said: "We are sorry for the inconvenience caused by this incident and would like to thank our customers for their patience today.
The company also warn that if water appears discoloured or aerated, residents should run the tap for five-10 minutes to allow it to settle to a normal flow.
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