Motorists have slammed a "daft diversion" that means they must travel 50 miles to get around 1,400 yards of roadworks.
The stretch of road in the village of Melbury Abbas, in north Dorset, is scheduled to be closed for five nights, to allow broadband fibre optic cables to be laid.
But drivers are being threatened with a £1,000 fine if they attempt to use the closed-off road.