The woman, who is in her 80s, was picked up from a day centre in Luton on Monday afternoon and should have been taken home to her flat at a care home.
Instead, she was found by a driver at 7am on Tuesday locked inside the Luton Borough Council minibus at one if its depots.
She was taken home shortly after being found and was not thought to have been injured.
The council has immediately put into action a policy of checking each passenger vehicle at the end of each day, which will remain in force until further notice.
Officials also began an internal investigation into the incident, while the driver of the minibus has been “suspended without prejudice”, a statement said.
Trevor Holden, the council’s chief executive, said yesterday: “Senior officers from the council met with our client’s family as soon as possible on Tuesday to make sure everything is being done for her continued welfare and to express how very, very sorry we are that this appalling incident took place.
“We are deeply shocked that something has gone so seriously wrong and we will do everything we can to find out exactly what took place and ensure it cannot happen again.”
A multi-agency adult safeguarding meeting organised by the council took place on Wednesday to investigate how the woman was stranded, why an alarm was not raised earlier and to ensure and immediate response.
In accordance with local policy, further meetings will take once the police have completed their investigations.
Mr Holden added: “We deeply regret the distress caused to everyone affected. Action will be taken.”