A suspected technical glitch has hit the NHS in Wales, leaving many staff unable to access computer systems.
The problem is affecting two data centres, the county's department of health said, adding that it is being dealt with "as a priority".
The British Medical Association (BMA) in Wales said it understood at least half of GP services have been affected.
Wales' Department of Health and Social Services tweeted: "There is currently a technical issue affecting two NHS Wales Data Centres. This is under investigation by the NHS Wales Informatics Service and is being dealt with as a priority."
Dr David Bailey, chairman of the BMA's Welsh council, said the issue would "no doubt cause a major disruption" to an already-stretched health service.
He said: "We are concerned to hear that NHS Wales' computer systems have been affected by a technical issue. BMA Cymru Wales has heard reports from some members that they are experiencing issues affecting emails, the internet and other systems, in both primary and secondary care.
"This will no doubt cause a major disruption to the NHS at a time when demand is already unprecedented.
"Welsh patients are those who will ultimately be affected, particularly if this results in doctors and other healthcare providers being unable to view medical records. We hope the situation is resolved quickly."