The nearside lane between Harcombe Cross and the filling station turn-off to Chudleigh will be a no-go area for traffic as workmen fix the fracture under the carriageway.
A SWW spokeswoman said the broken mains was not as deep-set as previously thought.
It was hoped the repair would be completed well before the start of tomorrow morning’s rush-hour.
Three overland pipes have been in place to carry water to homes affected by the burst.