At around 7.30am, a water main burst between A4127 Sudbury Court Drive and A409 Sheepcote Road, resulting in the closure of the A404 Watford Road in both directions.
The 18-inch main flooded the A404 to the depth of a foot and left an area of around 330 feet submerged in water, a spokesman for London Fire Brigade said.
Properties on more than 100 nearby roads had water supplies affected by the burst main, which was assessed by an Affinity Water technician at around 10am.
A spokesman for Affinity Water told getwestlondon : “We are sorry for any inconvenience caused by a burst on Watford Road, Harrow on Tuesday.
“The road is currently closed between Northwick Park roundabout and John Lyons roundabout, however access is being maintained for Northwick Park Hospital.
“We have isolated the damaged section of the burst water main and supplies should be restored to customers in the area.
“We would like to thank our customers for their patience whilst we carry out these repairs.”
Affinity Water also urged people not to use any electrical appliances that require a water supply, such as washing machines and dishwashers, during the disruption.
A spokesman for London North West Healthcare NHS Trust added: “A water main outside Northwick Park Hospital has caused flooding on the hospital site.
“If you are visiting Northwick Park Hospital, please allow extra time for your journey and take care while onsite.”
Nearby motorists faced delays of up to 20 minutes and seven bus routes were affected while Watford Road remained closed.
The flooding on Tuesday (January 16) comes two months after burst water mains in Wealdstone saw an entire street submerged in water , leading to the evacuation of more than 100 people.
Water levels in Cecil Road reached as high as 1.5 metres, with firefighters wading through the flood providing piggybacks to Harrow residents.
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