The A591 at Dunmail Raise, which runs through the Lake District, was badly damaged by storms in December.
Kier Highways has now began repairing the route after the Government made £40 million available to fix roads and bridges affected in Cumbria and Lancashire.
A 22-tonne excavator will be used on the site and a new concrete retaining wall will be built in the beck at the side of the collapsed part of the A591.
Once the wall has been built, a gap in the road caused by storm damage will be backfilled with crushed stone before a new road surface is laid on top.
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A new wall will also be developed using local stone in front of the retaining wall.
Transport Secretary Patrick McLoughlin said: "We are determined to reopen the A591 as soon as possible, and I am pleased the people of Cumbria can see we are making progress.
"Tourism is a vital part of the local economy, so fixing this road will make a real difference to the region."
Andy Withington, Highways England's asset delivery manager for the North West, added: "Preparatory work and some preliminary construction work are already underway, and we are starting work this week to reconstruct the collapsed part of the road at Dunmail Raise.
"We will continue to work closely with Kier Highways, Cumbria County Council and its contractors to do all we can to get the A591 repaired and re-opened."
Work on the second section of damaged road further north is also underway. A helicopter survey is due to take place shortly, which will provide detailed technical information about the stability of the scree slopes adjacent to the A591.
Other survey information is also being used to identify what repairs are required to storm drains, bridges and the road surface so that the A591 can fully reopen by the end of May.
(LM/CD)
