GPs, pharmacies and airports are among the UK services that have been affected by the global IT outage.
Some GP practices are struggling to access their records systems, which include online bookings.
There have also been long queues at airports and train services have also faced disruption because of a major problem with computer systems.
Some TV channels have been knocked off air, and stores such as supermarket Morrisons and bakery chain Gail’s say they have had problems with payments.
Problems started to emerge early on Friday when a number of businesses began to report IT problems.
Several firms have said the problems are due to a Crowdstrike cybersecurity software issue.
An NHS England 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.
“Patients should attend appointments unless told otherwise. Only contact your GP if it’s urgent, and otherwise please use 111 online or call 111.”
The spokesperson added there was no known impact on 999 or emergency services.
A spokesperson for the National Pharmacy Association said: “We’re aware that due to global IT outages that services in community pharmacies, including the accessing of prescriptions from GPs and medicine deliveries, are disrupted today.
“We urge patients to be patient whilst visiting their pharmacy.”
Long queues have been reported at airports, and Ryanair has said it is experiencing “potential disruptions” across its network.
The UK’s largest airport, Heathrow, said the outage was affecting “select systems”.
“Flights are operational and we are implementing contingency plans to minimise any impact on journeys.”
