Police say a car crashed and overturned on the eastbound carriageway between junctions 10 (Burnley, Padiham) and 12 (Nelson, Brierfield) at around 2am.
The motorway remains closed eastbound between the two junctions whilst emergency services work at the scene, close to junction 11.
Ambulance and fire crews are in attendance and the incident is being led by Lancashire Police.
Traffic is being diverted locally and has been coping well, but some delays are expected during the morning rush hour.
Motorists are being advised to expect delays and allow extra time for their journeys this morning.
A spokesman for National Highways said: "The closure is due to a collision that has occurred off National Highways' road network.
"The vehicle involved has overturned and come to rest on its roof.
"Lancashire Police are on scene leading this incident.
"National Highways have resources in attendance to assist with traffic management.
"Traffic is being diverted locally. Road users are advised to expect delays on the approach to the incident and allow extra time for their journeys."
Lancashire Police and North West Ambulance Service have been approached for further details.