The M1 is closed in both directions after a “suspicious object” was found underneath a bridge.
Traffic is being held up between junction 15 for Northampton and junction 14 for Milton Keynes.
The object was discovered beneath the motorway bridge at about 07:30 BST, Thames Valley Police said.
A bomb disposal unit has been seen travelling along the hard shoulder towards the scene and three fire crews are also in attendance.
The Ministry of Defence confirmed it had provided police with “explosive ordnance device assistance”.
Highways England has posted diversions but it is unclear when the road will reopen.
Trapped northbound traffic is being cleared by funnelling motorists through Newport Pagnell services.
National Express has added 45 minutes to travel times for services that use the motorway.
On Twitter, one stranded motorist wrote: “When you see a helicopter above you and kinda wish they were lowering a portaloo #m1 3 hours and counting…”
Another added: “Just heard a bang. Suspicious package dealt with maybe?”
#M1 b/w J15-J14 closures remain due to ongoing police-led incident-trapped traffic being released by being turned round-this will take time
— Highways England (@HighwaysEMIDS) September 19, 2017
— Jez (@Jez107M) September 19, 2017