The A35 is currently closed between the Stadium Roundabout in Dorchester to the Stinsford Roundabout.
It was closed around 1am as heavy rain fell overnight with Storm Chandra causing disruption in Dorset.
Traffic is reported to be heavy on the approach to Stadium Roundabout in all other directions with delays expected to be longer during peak times.
An amber weather warning is in place until 9am this morning.