Victoria Bridge in Kendal is being closed to west-bound traffic from this evening (Wednesday) due to flood damage on the river bed.
It had previously been closed for two days in early December, and re-opened - but today a specialist diving team found that the river bed had been eroded underneath one of the bridge pillars on the upstream side.
The A6 is being closed to west-bound traffic at Longpool roundabout with a diversion along Wildman Street and over Stramongate Bridge.
Traffic needing access to the Beezon Road Industrial Estate will be able to use Wildman Street and Beezon Road.
Victoria Bridge will remain open to east-bound traffic - although vehicles will travel over it on the opposite side of the road.
A spokesperson from Cumbria County Council said: "It is recognised that this bridge is of major importance to Kendal. Cumbria County Council is working with consulting engineers to agree the priority for repairs for bridges across Cumbria.
"Options for temporary and permanent repairs will be considered before any indication of timescales for the works can be announced."