The southbound carriageway of the M1 is currently blocked in between junctions 29 and 28 due to a multi-vehicle accident.
The incident - in which one car has overturned - happened at around 2pm and delays are already said to be significant.
The northbound carriageway was also closed for a time but this has now been reopened.
M1 CLOSURE UPDATE: Motorway reopens after multi-vehicle collision
An air ambulance has attended the scene in order to relay a casualty to hospital.
The fire, police and ambulance services are all also currently at the scene as well as the Highways Agency.
Normal traffic conditions are not expected to resume until at least 5pm and motorists are being advised to consider alternative routes.
Drivers caught up in the traffic should follow the hollow square diversion symbol.
More on this story will follow as and when we get it.