The M25 in Hertfordshire, closed after a fire on Tuesday, is not expected to reopen until late on Wednesday.
The van sits on the hard shoulder of the M25
The fire was between junctions 23 and 21a in a van carrying acetylene gas.
A police spokesman said the potentially explosive gas needed to stabilise after the heat. Both carriageways are due to reopen at about 1700 BST.
The rail link between St Albans and Watford, which was also closed because of the fire, was not expected to open again until Wednesday.
Diversions have been put in place by the Highways Agency.
Emergency services were called to the van fire south of St Albans on the anti-clockwise route at 1540 BST.
The fire was dealt with swiftly and no-one was injured but officers set up an exclusion zone round the scene when they realised the vehicle was transporting the potentially explosive cylinders.
The police spokesman said under fire service and health and safety regulations acetylene gas must remain undisturbed and monitored for 24 hours after a fire.
People have been asked to avoid the area through the night and for most of Wednesday.
"Travellers are asked to find alternative routes and to allow extra time for their journeys," he said.