Police closed the M1 motorway after a "suspicious package" was found under a bridge, according to reports. The motorway was closed in both directions, between junction 15 and 14, the Thames Valley Police said in a statement. They also urged motorists to avoid the area.
Highways England, a live traffic information service, said the motorway remained closed "due to ongoing police-led incident. Trapped traffic being released by being turned round. This will take time".
The traffic agency also urged the public to avoid the area as authorities are still dealing with the suspicious object.
"It was found underneath one of the motorway bridges between Junction 15 and 14 at around 7.30am this morning," Highways England said. "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."
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."
People stuck on the motorway have been tweeting about the delays and long queues.
A Twitter user, Stuart Paterson, posted a picture along with the caption: "Just heard a bang. Suspicious package dealt with maybe?" He also said that a bomb disposal crew was "on the way".
Another user, Liz Searle, said people were "making some new friends on the M1 this morning".
Users have also tweeted pictures of people playing football on the motorway.