Motorists were left stranded for several hours after a “suspicious object” was found under a bridge, forcing the closure of the M1.
The 11-mile (18km) stretch between junctions 15 at Northampton and 14 at Milton Keynes was shut both ways at 07:30 BST.
Thames Valley Police later said the object was not an explosive.
The northbound carriage has now reopened, but southbound remains closed, Highways England said.
It said “specialist” carriageway clearance work was still ongoing on the southbound side, but all trapped traffic has now been released.
Trapped northbound motorists were stranded for several hours before being redirected from the scene via Newport Pagnell services.
Footage earlier showed a bomb disposal robot pulling a bin bag off the hard shoulder. The bag appeared to contain a yellow substance.
Police said the liquid appeared to be a corrosive chemical and analysis was ongoing.
Thames Valley Police chief inspector Mark Lewis said there was “no evidence to suggest that this is a terrorist-related incident”.
The Ministry of Defence said it had provided police with “explosive ordnance device assistance”.
Insp Lewis said as there was no explosive device there had been no controlled explosion.
“The object contains a highly corrosive material which has leaked across lanes one and two, therefore the clear up process will take a significant amount of time,” he said.
Tim Mayer, a Conservative councillor from Coventry caught up in the delays, earlier said people had been trying to make the most of the free time.
“There’s a five-a-side football competition, a bit of rugby, a guy cycling up and down in the wrong direction,” he said.
“There are a few people with coffee in the car who have shared it around and others with some boiled sweets who have been generous.
“But the communication from police has been appalling.”
On Twitter, former England cricketer James Taylor joked he “might get the clubs out – perfect driving range”.
Highways England has a diversion in place for those travelling southbound via the A508.
At the scene – Mike Cartwright, BBC News
I’ve been speaking to people who have been stuck on the M1 for hours now.
Trevor Larkun has been stranded since 08:30, he says people are being “mostly patient” and chatting to other motorists.
“The mood is OK, we’ve had a few people come and deliver bottles of water and crisps,” he said.
“I’ve got sympathy for people I’ve seen walking down the hard shoulder with suitcases, presumably trying to catch a flight.
“The worst thing is you don’t know how long you’re going to be here for.”
Mr Larkun said that police officers told other motorists they may cut the central reservation at some point to release vehicles from the carriageway.