Thousands of motorists face travel disruption as a section of the M3 motorway has been closed in both directions after reports of "potential hazardous material on the road".
Bomb disposal units and emergency services are attending the scene between junctions 9 and 11 near Winchester.
Nobody has been hurt in the incident.
Police said they were called to the scene at 3.49am to reports of "potential hazardous material in the road".
The force said it is working Hampshire Fire and Rescue Service and the Explosive Ordnance Disposal team to establish exactly what the material is.
A statement said: "The road is still closed at this time, and we will endeavour to get the material cleared as soon as possible."
There are long tailbacks of vehicles caught up in the closure after Hampshire police shut the motorway in the early hours of Saturday.
Some motorists have taken to social media to express their frustration at being stranded.
But others preferred to look on the bright side.
George Vitelis, stranded at Winchester service station since around 9.30am, said the incident brought out "the best of the British spirit".
"It's quite a strange situation - there are babies having their diapers changed next to the M3, guys playing golf, people having picnics."
"The sun is shining, the weather is warm, I'm quite happy here to be honest. It would be different if I was stuck in the car with a load of screaming kids, but I'm not.
"It's moments like this that you see the best of the British spirit."