A section of the M3 motorway is closed in both directions after reports of “potential hazardous material” on the road, police have said.
Thousands of motorists are caught in queues on the motorway and surrounding roads.
Police said a bomb disposal team was at the scene of the closure between junctions nine and 11 at Winchester.
The incident was first reported at about 04:00 BST. Motorists have been advised to find alternative routes.
Hampshire Constabulary said officers were working with fire crews and the Ministry of Defence’s Explosive Ordnance Disposal (EOD) team to establish what the material is.
“We will endeavour to get the material cleared as soon as possible,” the force added.
Highways England has warned motorists to expect long delays as the motorway is expected to remain closed for several hours.
Motorists trapped by the initial motorway closure were able to leave the M3 by 07:00 BST, said Highways England.
At midday, queues on the M3 eastbound were three miles long and westbound they were two miles long as drivers waited to leave the motorway, it said.
It tweeted to warn football fans travelling to the Southampton match against Manchester United of the “heavy traffic”.
Hampshire County Council’s travel information provider Romanse said Winchester city centre and all routes in were gridlocked.
BBC reporter Toby Wadey at the scene
It’s absolute chaos on the roads. Motorists are switching off their engines and getting out of their cars.
It took me three hours to travel 10 miles. I’ve since abandoned a trip from Basingstoke to Bournemouth.
We were diverted off the M3 at the Winnall roundabout.
It took more than an hour just to get round the roundabout with people switching lanes and stopping in yellow junction boxes.
When I returned to an empty M3 northbound, the scene on the other side of the carriageway consisted of miles of queues, with thousands of motorists stranded.
Stranded motorists have taken to social media to express their frustration.
@neeshy_ tweeted: “Part of the M3 is closed. I’m going to be late. And I’ve just spilled hot coffee on myself. Shaping up to be a brilliant day”
Jim Hall said: “Stuck at Winchester Services M3 South, if the roads been closed since 4am why did they let us on if no way of getting off.”