Developers looking to build 270 houses, a convenience store and a coffee shop on land alongside the highway say "severe technical constraints" have forced them to abandon the idea of an underpass.
Instead a traffic light controlled crossing will be constructed to take walkers and riders from the new estate across the road into Garstang town centre.
The controversial development was originally turned down by Wyre Council in 2016 after more than 150 objections - a decision which was eventually overturned by a planning inspector.
Now the plans have gone back to the local authority because the intended tunnel for pedestrians and cyclists cannot be provided.
"On review of the technical capabilities of creating the underpass it has been identified that there are severe technical constraints to delivering it," says a report to the council.
"As such it is not a viable option to provide the underpass as the pedestrian link across the A6 to Garstang."
The old tunnel was blocked up after the former Garstang to Knott End railway line was shut down in 1965 and dismantled. The passage was back-filled to make it inaccessible.
But planners looking at the proposed housing development decided it could be unblocked and re-opened as a route for walkers and riders to reach the town centre without having to come into conflict with traffic on a busy road.
In the original planning application it was said that opening up the old tunnel would be feasible "meaning people wouldn't have to interact with traffic."
Now experts say it is no longer possible to unblock it and a road crossing will have to be installed instead.
"An alternative option has been devised to ensure there is safe pedestrian and cycle crossing of the A6 for the site, enabling safe sustainable transport routes to the town centre."
The project, linking the Garstang Marina area with the town, was first applied for in 2016. It went to appeal in January 2017 and was allowed by the inspector in April 2017.
Of the 270 homes earmarked for the 38.5 acre site 81 will be affordable properties. The scheme will also provide around five acres of public open space.