Luton Borough Council has said it is "deeply shocked" that a woman suffering from Alzheimer's was left locked on one of its minibuses overnight.
The woman, who is in her 80s, was picked up from the Hockwell Ring Day Centre in Luton on Monday.
She should have been taken back to her sheltered accommodation but was found on Tuesday morning at Kingsway depot.
Chief executive Trevor Holden said: "Something has gone so seriously wrong. Action will be taken."
The woman was discovered in the vehicle at 07:00 BST on Tuesday by a driver in the council's Passenger Transport Unit.
The council confirmed that an internal investigation began shortly afterwards, and the original driver of the minibus was suspended.
A meeting of relevant organisations took place on Wednesday to investigate how the woman was left on the minibus and why the alarm was not raised earlier.
A temporary procedure has also already been put in place to ensure every passenger transport vehicle is checked before the end of each day and this will remain in force until further notice.
Mr Holden said: "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.
"We deeply regret the distress caused to everyone affected."
Bedfordshire Police will also conduct an investigation and each of the organisation's involved will carry out internal inquiries.