Chase Farm Hospital in north London is among more than 6,000 properties affected by a burst water main.
Thames Water said the pipe in Enfield Road had been fixed and supplies would gradually be restored overnight.
Properties in the postcode EN2, including Enfield Town, have been badly affected. Problems have also been reported in EN1 and EN7.
Three water tankers have been supplying the hospital and bottled water has been handed out to residents.
Thames Water said they were alerted to the problem at 0400 BST and had since handed out up to 14,000 bottles of water.
A spokesman for the water firm said engineers had fixed the 16in main at about 2000 BST and were working to restore supplies.
He said it was a gradual process but it hoped all properties would have water by the morning.
Residents and businesses in the affected areas were warned they could have no water or low water pressure.
A hospital spokeswoman said operations had been cancelled.
She said: "Water tanks and the fire brigade are on site to ensure the main patient areas continue to receive water.
"Bottled water is being delivered to all areas."
The accident and emergency department remained open.