The incident happened between junction 29 and 28 where a car overturned at around 2pm.
Delays are said to be 'significant' and emergency services are on the scene.
The northbound carriageway was also closed for a time but this has now been reopened.
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.