Pharmacies have also reported issues with accessing prescriptions from GP surgeries and said this would affect the delivery of medicines to patients.
But NHS England said there was currently no known impact on 999 or emergency services as a result of the mass computer failure, blamed on a defective Windows update.
An NHS spokesperson said: “The NHS is aware of a global IT outage and an issue with EMIS, an appointment and patient record system, which is causing disruption in the majority of GP practices.
“The NHS has long standing measures in place to manage the disruption, including using paper patient records and handwritten prescriptions, and the usual phone systems to contact your GP.
“There is currently no known impact on 999 or emergency services, so people should use these services as they usually would.
“Patients should attend appointments unless told otherwise. Only contact your GP if it’s urgent, and otherwise please use 111 online or call 111.”
