Leeds Building Society has mislaid information containing the personal details of its 1,000-strong workforce.
The society has 70 branches throughout the UK
The West Yorkshire-based mutual, the seventh largest in the UK, has warned its employees to be vigilant as the data includes bank and salary details.
In a statement to its employees, it said there was no evidence the data had been taken from the building.
The information was lost when the human resources department was moved during a refurbishment of its Leeds head office.
Referring to this move, the statement said: "There has been a significant amount of disruption to achieve this in a short space of time and, whilst every care has been taken, some information is yet unaccounted for in the move from the 1st to the 4th floor.
"This contains the details you receive on your payslip, some of which is already in the public domain (i.e. on cheques, in the telephone directory, etc).
"We believe that the details are still in the building and are doing all that is possible to confirm this."
Searches of the head office building in Albion Street, Leeds, were continuing on Tuesday, a spokesman said.
The statement went on: "The missing information is not sufficient in itself to allow an unauthorised person to access a building society or bank account but we ask that you are vigilant in regard to the account into which your salary is paid.
"In the extremely unlikely event that any member of staff suffers a loss as a result of this situation, the society will make full reimbursement."
Managers were alerted to the problem last Thursday but staff were only told about it on Monday night.
The data loss comes amid raised concerns about the security of personal data following the disappearance of two discs from Her Majesty's Revenue and Customs containing the details of 25 million people.
Leeds Building Society is the UK's seventh largest with 70 branches in the UK and a call centre in Leeds. However, customers are not affected.