The northbound M1 has reopened after a nine-hour security alert caused by a “suspicious” object containing a corrosive material.
Thames Valley Police also said stationary vehicles on the southbound carriageway were being allowed to move past the scene after the object was found to have “no explosive element”.
Officers, who thanked the public for their patience during the closure of the M1 near Newport Pagnell services in Buckinghamshire, said chemical analysis on a substance found at the scene was continuing.
Chief Inspector Mark Lewis, of the Thames Valley Roads Policing Unit, said the M1 had reopened northbound but would remain closed southbound at junction 15.
The officer said: “There is no evidence to suggest that this is a terrorist-related incident. The object does not contain an improvised explosive device and therefore there has been no controlled explosion.
“However, 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.
“We apologise for the inconvenience caused to motorists but your safety, and the safety of our officers and the staff of our partner agencies, is paramount. We thank the public for their continued co-operation and patience.
“Motorists are advised to avoid the area and the force and Highways Agency continue to divert motorists away from the scene.”
Footage broadcast by the BBC showed a bomb disposal robot dragging what appeared to be a black rubbish bag off the hard shoulder, near a yellow substance on the road surface.
Tim Mayer, a Conservative councillor from Coventry who was caught up in the delays, said communication from police was “appalling” but 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.
The M1 was closed between junction 15 and 14 after the object was found under a bridge at around 7.30am, with police and fire services initially attending the scene.