Delays mean Dorset County Council will probably miss its target of reopening Beaminster Tunnel by the end of May.
Banks near the tunnel collapsed after heavy rain in 2012 killing Somerset couple Rosemary Snell and Michael Rolfe who were in a car crushed by the landslide.
The council is funding a £2.1million project using a process called soil nailing to stabilise the steep slopes near the tunnel.
Nails will be driven deep into slopes near the tunnel entrance and attached to a mesh which will hold the soil together.
But the council said heavy rain since the initial tragedy and the high demand for geotechnical engineers meant initial design and stabilisation work was completed later than expected.
Negotiations are also still taking place with landowners before the main repair work can start.
A council spokesman said: "Although considerable advanced preparation works continue, it has not yet been possible to start the soil nailing as programmed and it is now likely that the road will not be opened at the end of May.
"The planned completion date is likely to slip by three to five weeks but anything that can reasonably be done to reduce this delay will be done."
Peter Finney, Dorset County Council's cabinet member for highways, said: "We know this is devastating news for the community, and it is certainly not what we had foreseen happening.
"The design process is complex and does take time. It is extremely important that we get it right.
"We are doing as much other work on site as we can – including improving the drainage – and we will re-jig parts of the programme where possible to keep the work moving.
"We're getting the site prepared as far as possible given the current constraints so that when we have the green light we can get straight on with the main construction work."
Passing trade for businesses in Beaminster and Mosterton has been hit as motorists bypass both communities to avoid the tunnel closure.
Councillor Rebecca Knox, who represents Beaminster on the county council, said: "Residents, businesses and neighbouring parishes have experienced major inconvenience due to the tunnel closure.
"Any delay in the reopening is a massive blow to them.
"While the closure is in place, I am stressing the importance of getting other matters addressed, such as road surfacing, drainage issues, intrusive weeds in the hedgerows, and road signs."
