Up to 24 tonnes of flour and 200 litres of diesel have been spilt across the M5 in a crash.
Previously-bagged flour was left strewn across the southbound carriageway in a collision between a lorry and a car towing a caravan at about 04:30 BST.
One lane remains closed between junctions 14 and 15 in Gloucestershire amid a clean-up operation.
Highways England said the recovery operation and carriageway repairs could “take until mid-afternoon”.
Wiltshire Police said there were no reports of serious injury.
Jack Tappin, from Highways England, said the articulated lorry “ended on its side in lanes one and two” following the collision, damaging a safety barrier,
“While the car and the caravan have currently been recovered, the recovery operation for the lorry and repairs to the carriageway are possibly going to take until mid-afternoon,” he said.
“Further assessment will be needed once the lorry has been lifted to fully see the damage to the road surface, so this recovery estimate may change.”
The agency said there are currently delays of about an hour.
Motorists have been advised to avoid using the M5 southbound through Gloucestershire if possible.