London Fire Brigade said the water was more than one metre deep.
The brigade was alerted to the incident at about 04:58 BST and is at the scene with Thames Water.
Thames Water said: “We’re sorry for any inconvenience this may cause and will get the road opened back to normal as soon as we can.”
It added that once the main had been located it would be turned off and the water would be diverted into another main.
