A gas tanker caught fire on the M4 near Chippenham and Bath, which has caused long delays following a brief closure between junctions 18 and 17 in both directions. The westbound carriageway has now re-opened, while the eastbound has opened two lanes.
The lorry was carrying compressed gas. The accident could lead to the major delays on Easter bank holiday due to the dangers posed by the flaming gas tanker.
Wiltshire Police had already enforced safety precautions and warned passengers and drivers nearby to remain in the cars and roll their windows up until they receive further instructions as black fumes clouded the area of the accident.
A spokesman said: "Drivers are asked to remain in their vehicles and await instructions.
"An evacuation of the carriageway is likely."
Dorset & Wiltshire and Avon fire and rescue services were called in to battle the flames. The incident comes on one of the busiest periods of the year as millions of people make journeys across the UK for the long four-day weekend over Easter.
One female motorist reported being stuck in traffic for two hours despite not being near the tanker.
Jade Jones said: "I was travelling from Bristol to Bournemouth to meet a friend for her bank holiday birthday party. I needed to be in Bournemouth by 2pm.
"I left Bristol at 11.30 and hit the M4 at 11.55 and have been there ever since. We didn't see a fire engine arrive for 25 minutes. We're still an hour and 50 minutes away from our destination."
Highways England has urged motorists who were planning on using the M4 to seek other ways of getting to their destinations and said delays to journeys are likely.
The organisation tweeted: "For your own safety please stay in your cars. We are making preparations to get you moving asap. Pls be patient and await instructions."