The incident happened on Monday, between junction 33 (Capel Llanilltern) and junction 34 (Miskin), closing both carriageways and triggering a major emergency response, including an air ambulance.
The collision was first reported shortly after 3.50pm, with the westbound side of the motorway closed between J33 and J34.
Traffic was diverted off and back on via the slip roads, while lane three was also closed eastbound.
South Wales Police described the incident as a “serious collision” and warned that the road would remain shut “for some time,” urging drivers to avoid the area, use alternative routes and allow extra time for journeys.
The force thanked motorists for their patience as queues quickly began to build, reaching as far back as junction 28.
Traffic Wales South confirmed that both carriageways were closed as the seriousness of the incident became clear.
They labelled it a “serious incident” and warned that the motorway was likely to remain shut for an extended period.
The closure created major traffic issues across the surrounding road network.
Severe delays were reported on the A4232 northbound from Leckwith, the M4 westbound from J32 (Coryton) and the M4 eastbound from J35 (Pencoed) as drivers sought alternative routes.
AA cameras showed queues moving at around five miles per hour and delays of more than 40 minutes in the worst-affected areas.
Eyewitnesses described seeing a large emergency services presence and “someone lying on the ground,” with photos showing an air ambulance on the carriageway.
Images also showed multiple emergency vehicles surrounding the central reservation.
By around 6.40pm, Traffic Wales South confirmed that both carriageways between J33 and J34 had reopened.
However, they advised that lane three would be put in place in both directions due to damage from the collision.
Shortly after 7.20pm, AA cameras indicated that traffic had returned to moving smoothly through the area, with the worst of the congestion cleared.
At the time of publication, no further details had been released about the number of vehicles involved or any injuries.
Enquiries with emergency services remain ongoing.
While the full circumstances of the collision are still under investigation, authorities have confirmed that this was a significant incident requiring a major emergency response.
