A bomb disposal robot was called to the M1 after a 'suspicious object' was found under a motorway bridge.
The road is closed in both directions between junctions 14 and 15 in Newport Pagnell, Buckinghamshire, causing huge tailbacks - and police are urging motorists to stay away.
One Twitter user said the lockdown was caused by a "smoking package containing yellow bubbling liquid".
There are reports a m1controlled explosion was carried out by emergency services, but this has not been confirmed by police.
Most Read
Motorists said they had been sitting in queues for four-and-a-half hours - and some even started playing football to pass the time.
In a statement, Thames Valley Police said: “We are urging the public to avoid the M1 as we are currently dealing with a report of a suspicious object.
Don’t miss
“It was found underneath one of the motorway bridges between junctions 15 and 14 at around 7.30am this morning.”
Social media users said they had been trapped in tailbacks for four hours.
One motorist said: "We don't have too much information but we're trapped right in the middle of it.
"The helicopter has come and gone a few times and cars ahead of us are being turned away. We're stuck about 1.5 miles from Newport Pagnell services."
"At first we heard it was a chemical incident but then a suspicious package."
The police added: "The road closures are likely to be in place for some time while Thames Valley Police and the fire service investigate the matter."
Social media photos show motorists leaving their cars and walking away.
Buckinghamshire Fire and Rescue Service said: "Firefighters are assisting Thames Valley Police with their investigation into a report of a suspicious object found underneath a motorway bridge, M1 southbound between junctions 15 (Northampton) and 14 (Milton Keynes).
"One appliance and crew from Newport Pagnell, one from Broughton, one from Great Holm, the Environmental Protection Unit from Aylesbury and two officers are attending."