The glitch at NHS Greater Glasgow and Clyde, and in particular the lack of any successful back-up system, prompted Health Secretary Alex Neil to order a review of IT systems across the NHS in Scotland.
It is understood the problem arose when the network became corrupt and stopped granting access to users. By last night, a solution had not yet been found and patients became increasingly critical of the health board.
John Greer, from Glasgow, said he had waited six months for an appointment, only for it to be cancelled. He added: "The place was in utter chaos, I can't believe there is no back-up."
The board's two main software suppliers - Microsoft and Charteris - were working to resolve the issue last night.
A total of 485 outpatient appointments, 14 planned inpatient procedures, 43 day cases and 48 chemotherapy treatments have been postponed so far.
Jean Turner, executive director of the Scotland Patients Association, warned that delays in appointments could have significant consequences. "I do hope that no patients will be put in danger because of this," she said.
The health board said last night additional resources would be brought in to ensure affected patients would be treated as quickly as possible. Mr Neil said in parliament he had instructed a "robust review" of NHS IT systems in Scotland.
Robert Calderwood, chief executive at NHS Greater Glasgow and Clyde, said: "I apologise unreservedly for the inconvenience this has caused to our patients who have had their procedures postponed and I will ensure everything possible is done to get their treatment carried out at the earliest possible opportunity. Our senior management team, IT specialists and clinical leads have been working around the clock and will continue to do so until this issue is resolved."
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