The M3 continues to be closed in both directions. Police were called at 3.49am to reports of potential hazardous material in the road.
Officers are working alongside partners from Hampshire Fire and Rescue Service and the Explosive Ordnance Disposal (EOD) team to establish exactly what it is.
Nobody has been hurt in the incident.
Witnesses sought after flammable- not explosive- material appears to have been thrown from motorway bridge on two consecutive weekends
Motoring groups are calling for better information for road users after thousands of motorists were trapped on the M3 in Hampshire.