A terror alert, which closed the M6 toll on Thursday, was caused by a "health improvement aid for smokers", police said.
Officers responded after a report at about 8.20am from "a genuinely concerned member of the public" who was on a coach travelling from Preston to London, .
The passenger saw vapour coming out of a bag as the vehicle was near the M6 toll plaza at Weeford, near Lichfield.
The bus pulled on to the hard shoulder until police arrived. Armed officers then cleared the scene and led the passengers off the Megabus coach one by one.
The passengers, including at least one young boy, were taken to a cordoned off area of the motorway and surrounded by police.
The single-decker coach left the scene, escorted by a number of police vehicles, shortly after 12.35pm.
A spokeswoman for Megabus, which is operated by Stagecoach, said 48 passengers were booked on the service and all were safe and well: "We are assisting police with their inquiries into an allegation made against a passenger who was travelling on the 05.10 Preston to London service," she said.
"There were 48 people booked to travel on board the service, which was due to arrive at Victoria Coach Station in London at 10.55am."
Several other cordoned off areas could also be seen, along with what appeared to be decontamination units.
Military personnel, police dog handlers, firefighters and other specialist units were also at the scene.
The incident shut the motorway toll road in both directions for more than four hours.