A computer disk containing the names and addresses of more than 11,000 teachers has gone missing in the post.
The General Teaching Council (GTC)'s letter to teachers said it went missing after being sent from Rotherham via Parcelforce to its Birmingham office.
A spokeswoman said it held the details of teachers who had filled in a form updating their registration details.
She said that no financial information, National Insurance numbers or dates of birth were included.
The GTC, which is the professional body for teaching in England, said the disk had been sent using Parcelforce's tracking system but did not arrive when expected.
The CD had been sent from the GTC's data contractor in Rotherham.
The firm's vehicles and premises were searched but the disk has not been found.
The spokeswoman said "all the evidence available to us suggests that it is still mislaid within the courier system".
The GTC said the information had been encrypted, but added that it had strengthened security measures on its records following the loss.
The problem was first spotted on 12 September.
GTC Registrar Alan Meyrick said: "We are taking this issue very seriously.
"We know that incidents like this are concerning and deeply regret any worry caused to teachers involved."
Tim Philby, a primary school teacher in East Yorkshire, said he was shocked when he read the letter.
He added: "Opening and reading the letter I was shocked, but then came the realisation that I can't do anything about it and I just hope that nothing untoward happens.
"There is a fear of my information being taken by someone I do not want it to be given to and then used for different means that I want them to."
Teachers concerned they may be affected can check by calling 0870 001 0308.