The M4 is partially blocked between Junction 16 for Swindon West and Junction 17 for Chippenham after a lorry overturned at about 8am today.
Traffic heading west is delayed with queues of 10km (6.2 miles) reported.
Highways England warns that traffic conditions may not return to normal until this afternoon.
It is not thought that anyone was hurt when the container lorry, being driven by a 25-year-old man, overturned about a mile west of Junction 16.
The M4 westbound was being fully closed between junctions 16 and 17 from 12.30pm for up to an hour to help emergency crews right the lorry.
A police spokesman said: "Emergency crews are hoping to have the vehicle, which is lying on its side, back on its wheels by 1.30pm/2pm at the latest today.
"Drivers are asked to be patient or find an alternative route; diversions are in place."
Traffic is now heavy on the eastbound M4 in the area as drivers slow down to see what is happening.
1 lane closed #M4 westbound between J16 and J17 due to an overturned HGV. Please take care on approach.
— Highways England (@HighwaysSWEST) September 9, 2016
We have been advised of 10 km queues on the westbound M4 junction 16-17 as a result of overturned vehicle. Advise plan alternate route.
— Tri-Force Police (@Tri_Force) September 9, 2016
Very heavy traffic eastbound M4 17-16. No incident this is purely rubbernecking. #keepyoureyeontheroad
— Tri-Force Police (@Tri_Force) September 9, 2016
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