Bomb disposal units have been called out after emergency services shut a section of the M3 motorway, causing travel misery for thousands of motorists.
Scores of vehicles are said to be caught up in the closure after Hampshire police shut the motorway between junctions 9 and 11 near Winchester in the early hours of Saturday.
Police said they were called to the scene at 3.49am to reports of "potential hazardous material in the road".
Nobody is said to be hurt.
Live at scene. Bomb disposal units in place. If you look closely a person in special clothing is bent over inspecting something under bridge pic.twitter.com/mdtHKjCm4h— BBC Radio Solent (@BBCRadioSolent) September 23, 2017
A police spokesman said he was unable to confirm the nature of the cargo, whether it was place deliberately on the carriageway or whether it was an accident.
A force statement added: "Our officers are working alongside partners from Hampshire Fire and Rescue Service and the Explosive Ordnance Disposal (EOD) team to establish exactly what it is.
"The road is still closed at this time, and we will endeavour to get the material cleared as soon as possible.
"Thank you for your patience, and please keep an eye on our social media channels for further updates."
Passengers stuck on the motorway captured footage of some motorists driving the wrong way down the hard shoulder in order to escape - a traffic offence which routinely carries a fine.
Witness Jenna Keating, 31, a writer from Windsor, said: "There are lots of people stranded here, people have got out of their cars to stretch their legs and things, but some are just turning round and going back down the hard shoulder.
"We actually spoke to police about this to see whether this was something we could do in the circumstances and they said 'absolutely not'.
"They also said the delay had been quite well advertised, although none of the four of us in the car saw anything until we were in it.
"We're one of the lucky ones, we've only been here for 40 minutes although we've basically only crawled forward into space probably left by those who are trying to turn around.
"I can't think it's a good idea to use the hard shoulder - there have been a couple of unmarked vehicles with sirens blaring coming up and using it, so it's pretty dangerous if there are cars coming in the other direction."