Welsh government says there were no data security issues
The Welsh government says all NHS computer systems are now back working, although there may be a backlog affecting some areas.
The problem has stopped doctors from accessing patient records and affected email and intranet services.
A Welsh Government spokesperson said: "The NHS Wales Informatics Service have confirmed that all systems are now back although there may be a backlog affecting some areas.
"They have assured us that there were no data security issues. The cause of the technical issue that affected two NHS Wales Data Centres remains under investigation."