The Beaminster Tunnel in Dorset is unlikely to be re-opened for several months.
It was closed earlier this month after a landslip that killed two people.
Engineers have found that the slope at both ends of the tunnel is extremely dangerous and liable to further collapses. Dorset County Council has brought in experts to find the best way forward.
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The M32 will be closed between Junctions 1 and 2 between 8pm and 6am on July 26/27 and 27/28. Diversions are in place.