The A34 southbound between the A303 and the junction with the M3, is completely backed up this afternoon.
The traffic backs up to the Winnall Roundabout, the likely cause being The Boomtowm Festival which runs in Winchester from 11th-14th August 2016.
There are currently delays of around 40 minutes, the traffic is not expected to clear until late this afternoon.
Drivers are advised to avoid the area and take an alternative route.