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.
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 which use the motorway.
The Ministry of Defence has been contacted for comment.