Confidential health records of more than 38,000 patients have gone missing after a computer back-up tape was lost by a courier firm, an NHS trust said.
The Isle of Wight's Sandown Health Centre sent its records to a specialist firm for its software to be checked.
But the tape, containing records of patients dating back to July 1996, failed to arrive when it was sent back using courier firm City Link in March.
The Isle of Wight NHS Primary Care Trust fears "it may be lost forever".
A City Link spokesman said: "Naturally we are concerned about the loss of our customer's consignment and a rigorous search continues.
"An investigation is under way and we are doing everything in our power to resolve this situation."
A spokesman from the trust explained the back-up tape was sent away to ensure that it could be used effectively to restore information on the health centre's computer if it was damaged in a fire or other system failure.
He said the back-up tapes were more secure than data stored on CD-ROM and the health centre still had a copy of all the data so patients would not be affected.
"The risk of the tape being misused is extremely small," the trust spokesman added.
"The tape requires specialist computer equipment to run it and the data is password-protected.
"Highly advanced computer skills and/or access to a specialist programme only normally used by GPs and the data verification company are needed to make any sense of the information on the tape."
The PCT and the health centre, which has been contacting patients to advise them of the loss, has set up a telephone helpline to address people's concerns.
The hotline number 0845 602 6834 will be open between 0800 and 2000 BST Monday to Friday.