Police have partially reopened a section of the M1 after a bomb squad was called to investigate a "suspicious object" under a bridge.
The motorway was shut between junction 14 at Milton Keynes and junction 15 at Northampton after the item was discovered at about 7.30am during the morning rush hour.
Bomb squad officers were at the scene throughout the day - but shortly after 5pm, police confirmed the object did not contain explosives and no terror link was suspected.
Thames Valley Police said "highly corrosive material" had leaked across two lanes of the motorway - meaning further misery for evening rush hour drivers in the area.
The northbound carriageway has reopened and some vehicles stranded on the southbound carriageway have been able to move away from the scene - but the southbound carriageway remains closed at junction 15.
Police warned the clean-up operation would take a "significant amount of time".
Images on social media showed a robot being used to investigate what appeared to be a bag or sack at the edge of the hard shoulder under the bridge.
A smear or stain appeared to have spread partly across the hard shoulder away from the bag.
The Highways Agency warned motorists to expect delays of at least an hour and recommended they use alternate routes.
Chief Inspector Mark Lewis, from the Joint Roads Policing Unit for Thames Valley and Hampshire, 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. Chemical analysis is ongoing to ascertain what the substance within the object is.
"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.
The Highways Agency said those already trapped in the traffic jam were being turned around, but the process may take some time.
Many motorists reported being stuck in traffic on the motorway for up to six hours and some pictures on Twitter showed people playing football and other sports on the clear side of the carriageway.
Other images showed people bringing food and drink to those trapped in the jam.
The A508 and A5 were also heavily clogged.