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 a motorway bridge at about 07:30 BST, Thames Valley Police said.
Motorists are urged to avoid the area as the road is expected to be closed “for some time”, the force added. Highways England has posted diversions.
Buckinghamshire Fire and Rescue Service has sent three crews to the scene.