The motorway is closed in both directions at junction 19 (Heywood), near Manchester.
Delays of up to 90 minutes are being reported on the westbound carriageway, with traffic now building up as far back as junction 22 (Ripponden).
Highways England said traffic is being diverted eastbound and westbound via the exit and entry slips at J19.
Drivers are advised to allow extra journey time to complete their journey if travelling in these directions or to consider an alternative route.
Normal traffic conditions are currently expected to resume by around 1.45pm.