A busy motorway has been closed in both directions after a "suspicious" chemical substance was found under a bridge.
Police and firefighters were called after the "suspicious object" was discovered during Tuesday's morning rush-hour.
Motorists were urged to avoid the M1 near Milton Keynes, Bucks., as police refuse to comment on whether the investigation was thought to be linked to terrorism.
Three fire engines and an environmental protection vehicle were sent to the scene between junctions 14 and 15.
Diversions between Milton Keynes and Northants were expected to be in place for "some time", said Thames Valley Police.
A statement on the AA website says: "Trapped traffic is being held. Police and Fire are investigating an unknown chemical substance on the road.
"Detour in operation - follow the hollow diamond symbol. Strategic diversion advised to use the M40 from M25 for the Midlands and North."
A spokesman for 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.
"It was found underneath one of the motorway bridges between Junction 15 and 14 at around 7.30am this morning.
"The M1 is currently closed in both directions between Junction 15 and 14, and diversions are in place in both Milton Keynes and Northamptonshire.
"The road closures are likely to be in place for some time while Thames Valley Police and the fire service investigate the matter."
People have been waiting for so long for the M1 to re-open that they have begun to play sport in the road:
Some are playing Frisbee
And one man has been forced to share his provisions with thirsty motorists:
Been in the same spot for more than 4 hours - starting to give out my coke cans to people who need a drink, glad a bought a multipack #m1— Taran (@TaranjeetTJ) September 19, 2017
Are these people hoping to catch a flight by walking?
People have been reporting waiting times of up to six hours
The mystery chemical was found near where eight people died in a horror M1 crash in August.
Six men and two women died and four others including a five-year-old girl were taken to hospital after the crash involving two lorries and a minibus near Newport Pagnell, in Buckinghamshire.
Both lorry drivers have appeared in court charged with causing death and serious injury by dangerous driving.
Ryszard Masierak, 31, of Evesham, Worcestershire, is accused with eight counts of causing death by dangerous driving, four counts of causing serious injury by dangerous driving and eight counts of causing death by careless driving while over the prescribed alcohol limit.
David Wagstaff, 53, of Stoke-on-Trent, is charged with eight counts of causing death by dangerous driving and four counts of causing serious injury by dangerous driving.