Kelston Road will remain closed for at least the next six months as Bath and North East Somerset Council reveals the extent of the stability work needed.
The local authority said such a lengthy closure was not something it had "taken lightly" but tests have shown that the ground is slipping by as much as seven metres below the road surface.
It has previously said that allowing people to use the A431 could prove fatal to any road users because of the risk of a landslip.
A statement released this afternoon said: "Following the closure of the Kelston Road in mid-February, the ground remains unstable and the landslip is still active.
"The council is sparing no effort to establish exactly what underground damage is being caused by water moving deep underground and destabilising the road.
"Without understanding the exact path taken by the water, we are not in a position to resolve it in the long-term."
It added: "Due to the complexity of this problem, the council will need to complete an analysis and design of permanent works to the best repair in terms of cost, effectiveness and completion time.
"Based upon a comparison with similar landslips elsewhere in the country the works will take at least six months to complete."
B&NES has said specialist engineers have been looking underneath the road, which has been damaged by water running off nearby fields - caused by the recent heavy rain.
They need to drill up to 20 boreholes to make the road safe and at the moment any movement under the road is being monitored.
A spokesman said it was not safe for cars, bikes or pedestrians to be on the stretch of road, which is the main route between Bath and Bitton.
"Closing the road is not a decision we have taken lightly," he said. "We understand the impact on people's journeys - but the safety of the travelling public is absolutely paramount and until there is a solution in place we cannot consider allowing vehicles, cyclists or people on foot to use this stretch of highway.
"It is simply too dangerous."
Temporary electronic road signs have been installed at the approach to the A431, warning people about the closure.
Diversions are in place and people coming into Bath should go via the A431 to Willsbridge, A4175 towards the Hicks Gate roundabout, A4 Keynsham bypass, towards the Globe roundabout, A36 Lower Bristol Road, across Windsor Bridge Road, rejoin A431 at Newbridge Hill.
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