Repair work to a Shropshire bridge, which closed to traffic when masonry fell from it, started on Monday.
Ludford Bridge, in Ludlow, closed when parts of it fell into the River Teme on 4 January. It has since reopened to pedestrians.
Shropshire Council believes frost got into the ancient monument.
A council spokesman said the work, estimated to cost £90,000, could not begin earlier because of time needed for the design to be approved.
The work is scheduled to last for nine weeks.
The spokesman said on Monday work to erect a "complex" scaffold platform began to enable contractors to safely access the damaged part of the bridge.
He said the repair scheme will involve taking down areas of unstable stone masonry from alongside the existing area of collapse and then rebuilding the damaged part.
Until the bridge is reopened to traffic diversions are in place.
The bridge is an ancient monument which was the site of a famous battle in the War of the Roses in 1459.
Martin Taylor-Smith, Shropshire Council's Cabinet member for transport and IT, said: "It is appreciated that the closure of the bridge is causing inconvenience and every reasonable effort will be made to complete the work and reopen it as soon as possible."