REPAIR works to the Beaminster Tunnel have been delayed and the route will not open in time for its original May deadline.
Rosemary Snell, 67, from Misterton, and Michael Rolfe, 72, from Fivehead, died when their car was engulfed in soil after a landslip at the tunnel last July. The road has remained closed since.
The closure has caused serious problems for businesses in Beaminster.
Dorset County Council says its £2.1 million scheme to make the tunnel safe has been delayed because geotechnical engineers have been in high demand following recent heavy rainfall.
The companies contracted by Dorset County Council to test the tunnel site and design the stabilisation works have both completed their work later than originally expected.
Councillor Peter Finney, cabinet member for highways and transportation at Dorset County Council, 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."
Councillor Rebecca Knox, Dorset County Council division member for Beaminster, said: "Residents, businesses and neighbouring parishes have been majorly inconvenienced due to the tunnel closure, and any delay is a massive blow.
"The delay is very disappointing for all of us and I hope the highways project team will be able to get on with all aspects of the work as each matter is resolved to get the road reopened as soon as possible."
According to the council, the planned completion date for late May is likely to slip by three to five weeks.