A "multi-vehicle collision" was initially reported by National Highways at around 12.30pm, which led to traffic being stopped on the southbound carriageway between Junction 13 for Penkridge and Junction 12, Cannock.
While traffic was released and two lanes reopened shortly afterwards, the agency was warning of long delays.
At their peak, queues reached six miles long with delays of 45 minutes as motorway traffic cameras and National Highways' traffic map both show queuing traffic as far back as Junction 14, Stafford North.
At around 1.30pm National Highways said: "Delays have reduced to 20 minutes on approach with five miles of congestion. Thank you for your patience."