Bomb disposal experts are preparing to carry out a controlled explosion after finding “a number of ageing detonators” beside a motorway.
The A1 at Catterick in North Yorkshire has been closed since this morning after a member of Army staff reported hearing an explosion in the early hours.
The detonators were found during a search of the motorway area, but police said they were not the cause.
Officers said a “credible source” at Marne Barracks had reported the sound of an explosion at 3am.
However, no one in Catterick village has backed up the report.
Police said they could not rule out that the reported explosion occurred “within the boundary of the barracks”.
Catterick is home to the Army’s Northern Super Garrison, covering 20,000 acres of training land.
Every infantry soldier undergoes phase one and two training at the sprawling site.
North Yorkshire Police said the road closure was a precaution while they investigated the area around the motorway,
“All security precautions must be taken to protect members of the public until we find out exactly what has occurred,” said the force.
Six miles of motorway are affected and the road will reopen “when it is safe to do so”, police said.
Both slip roads to the area are shut and diversions are in place, with motorists warned to avoid the area.
The Highways Agency said the closure was between Junction 51 at Leeming Bar and Junction 56 at Barton.
Newcastle United football fans were expected to be among those affected as they headed south to play West Ham.