Hundreds of homes in north London have been left without water due to a faulty valve at a reservoir.
Affinity Water said it had identified the failure at Arkley Reservoir, Borehamwood, as the cause of the supply interruptions.
A spokesman said the valve had been bypassed but it could take several hours for supplies to return to normal.
Large areas around Barnet were affected including the N2, N3, N12, N14, N20, EN4 and EN5 postcodes.
A wider area is expected to suffer from low pressure as water is restored in the area.
Affinity Water said customers should expect discoloured water, caused by small particles of iron entering the water stream, and cloudy aerated water - neither of which are harmful.