The terror alert which closed the M6 toll road was caused by a "health improvement aid for smokers", police said.
Armed officers responded to a genuine security alert from a concerned member of the public on board the coach travelling from Preston to London, Staffordshire Police said.
"The information received concerned a report of vapour escaping from a bag which on investigation turned out to be a health improvement aid for smokers," a force spokeswoman said.
No-one has been injured, there is no danger to passengers and no-one is being treated as a suspect after armed officers evacuated the Megabus coach, police said.
Officers responded "to a genuine report from a member of the public", the force spokeswoman said.
"Given the credibility of the information we received, we responded swiftly and proportionately. We can now confirm that, whilst this was a genuine security alert, the significant concerns reported to us were unfounded."
She went on: "It's important to state that no criminal offence has been committed and no passenger or any other member of the public is being treated as a suspect.
"Our utmost priority was the safety and security of the public and whilst on this occasion the incident was resolved peacefully without event, we want to encourage the public to remain vigilant and report any suspicious activity. We would like to apologise for any inconvenience and hope that the public understand that we have our duty to safeguard public safety."
Police received a report at about 8.20am on Thursday from "a genuinely concerned member of the public" who saw vapour coming out of the bag as the coach was near the M6 toll plaza at Weeford, near Lichfield, the force spokeswoman said. The incident shut the M6 toll road in both directions for more than four hours.
A spokeswoman for Megabus, which is operated by Stagecoach, said 48 passengers were booked on the service and all were safe and well.