Firefighters have worked through the night after a van caught fire and the flames spread to heathland in the New Forest, creating a smoke plume visible for miles.
The fire broke out on the A31 near Ringwood, Hampshire, shortly after 17:20 BST on Sunday, forcing some motorists to abandon their vehicles .
Twenty-six fire crews alongside water carriers and specialist off-road units were sent to the scene as the flames quickly spread.
National Highways said the dual carriageway remained closed in both directions between the M27 at junction 2 for Ower and the A338 at Ringwood.
Smoke from the fire drifted across both sides of the Solent, prompting fire crews to urge people to keep doors and windows shut.
The fire service said a large hose had been laid from Hightown Lake, just east of Ringwood, to supply water to crews tackling the blaze.
In some areas, the hose crosses tracks and passes through private property.
Residents have been urged not to drive over it, as doing so could damage the pipe and disrupt the water supply needed to fight the fire.
Police have advised motorists who had to leave their vehicles behind to call 101 with their number plate details so arrangements can be made for them to get them back.
Traffic travelling westbound is being diverted onto the A326 towards Ashurst Bridge and then onto the A338 to the Ashley Heath roundabout to re-join the A31.
Traffic travelling eastbound is being diverted onto the A338 towards Bournemouth.
Hampshire & Isle of Wight Fire & Rescue Service said it had received more than 160 emergency calls reporting the initial vehicle fire.
The New Forest is home to a large variety of wildlife, including wild ponies, deer, rare birds, reptiles and insects that live in it abundant woodland, heathland and wetlands.
Related internet linksHampshire and Isle of Wight Fire and Rescue Service