The General Teaching Council for England (GTC) has lost the personal details of 11,420 teachers after losing a computer disk in transit.
It has apologised to teachers and strengthened security measures on records on its Teaching Register, following the loss of the disks in transit to the GTC.
The data consisted of electronic copies of registration update forms recently submitted by 11,423 teachers. The disk was being sent by the GTC's contracted data processor to the GTC's office in Birmingham.
Each form included a name, address and teacher reference number. It also included any changes that teachers had written on the form. The disk does not contain National Insurance numbers or financial data.
"The GTC is treating this matter with the utmost seriousness. The disk was sent via a recognised courier using its full tracking service, but did not arrive at the GTC when expected. An extensive search of the courier's vehicles and premises has not yet resulted in the disk being found," said the GTC.
It said the disk was encrypted, "so not readily usable by a third party and all the evidence available to us suggests that it is still mislaid within the courier system", added the GTC.
The GTC has informed all teachers involved. If a teacher did not fill in a registration update form during July or August, they are unlikely to be affected.
The GTC has been moving over to secure electronic data transfer, which avoids the potential problem of a parcel being mislaid in the system.
GTC registrar Alan Meyrick said, "We have taken all possible steps to protect the integrity of the Teaching Register and are working to ensure that there will be no similar incident in the future.
"Because we recognise that no encryption system can ever be entirely infallible, we have taken urgent steps to put additional security measures in place for affected records. We have written to teachers affected to apologise, and to assure them that steps have been taken to prevent records being accessed or misused in any way."