Emergency services responded to a crash involving a motorhome and a car.
The incident happened at about 14:10 BST on Thursday inside the Saltash Tunnel, which lies on the Cornwall side of the Tamar Bridge.
Devon and Cornwall Police said the ambulance service were also in attendance, with minor injuries reported.
National Highways confirmed the road has since reopened, but there are current delays of 25 minutes and two miles of congestion in both directions of the A38 on the approach.
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Car and Motorhome Crash Causes Saltash Tunnel Delays