The Scottish Ambulance Service has admitted that a package containing contact information has been lost.
The computer disc from its Paisley Emergency Medical Dispatch Centre (EMDC) was lost by the courier, TNT.
The disc contained a copy of records, including some phone numbers and patient names, of 894,629 calls to the Paisley centre since February 2006.
It was encrypted and password protected and an ambulance spokesman said it would be extremely difficult to access.
The disc was sent on 9 June to MIS Emergency Services Ltd in Manchester.
This is not people's private medical records
Nicola Sturgeon Health secretary
MIS supplies the computer system used in the Scottish ambulance service's three EMDCs.
TNT said it could not locate the package.
Health Secretary Nicola Sturgeon said: "This is call centre information that the ambulance service takes from people calling in.
"It has no personal information that could be misused by anybody.
"It will contain some addresses, some patient names and in some circumstances telephone numbers.
"This is not people's private medical records."
She added that it was a very serious breach on the part of TNT and the government would keep closely involved in follow-up discussions.
A spokesman for the ambulance service said the disc information was to be used in the development of command and control systems.
Newsnight Scotland asks why a courier company was used to transfer the data
Pauline Moore, acting chief executive of the Scottish Ambulance Service, said: "The information stored on the disc is fully encrypted and it would be extremely difficult to access any names or addresses.
"We have established a phone line for anyone who may have concerns to call and discuss them with us."
She added: "The secure management of patient data is vitally important.
"We have followed recommended guidelines for the transfer of data and it is disappointing and regrettable that this has happened. We are currently addressing the issue with TNT."
Labour's Margaret Curran said the loss of such data would be "deeply worrying" for many Scots.
She added: "Nicola Sturgeon must now take decisive action to reassure the public that everything is being done to retrieve the information.
"The Scottish Government has delayed for two weeks before admitting the loss of almost 900,000 personal records. Nicola Sturgeon must now make an emergency statement to parliament setting out all the facts."
David McLetchie MSP, the Scottish Conservative chief whip, said: "We need to know more than we are being told, and particularly whether or not any patients or families who might be affected by this breach have been notified."
In order to protect the 999 system from inquiries by the public, the ambulance service has set up a contact number (0800 783 4914) for anyone who has any questions about the loss.
The service has asked callers not to phone 999 with any inquiries.
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