Most GP practices in England have been disrupted by the global IT outage, the NHS has said.
GPs have been struggling to access their records systems, which include online bookings, and pharmacy services such as accessing prescriptions have also been affected.
The IT outage has also led to long queues at airports and has knocked some TV channels off air.
The exact cause of the problem is unknown, although several firms have blamed 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 British Airways says some its flights are “likely to experience disruption”.
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.”
Train services are still running, although the National Rail Enquiries website has warned “widespread IT issues across the entire network”.
It said this could potentially lead to “short-notice train cancellations”.
“Additionally, other key systems, including real-time customer information platforms, are also affected,” it added.
It said services run by Gatwick Express, Great Northern, Southern and Thameslink may be affected by alterations and cancellations.