Most GP practices in England have suffered disruption as a result of the major global IT outage, causing problems with booking appointments and issuing prescriptions.
But NHS England said there was currently no known impact on 999 or emergency services as a result of the mass computer failure.
Thousands of doctors' surgeries have been affected after the widely-used EMIS appointment and patient record system went down.
Global IT outage: Follow live
Pharmacies have also reported issues with accessing prescriptions from GP surgeries and said this would affect the delivery of medicines to patients.
A spokeswoman for NHS England 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."
EMIS Web enables GP practices to book appointments, examine records and includes a clinical decision support tool as well as helping with admin.
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What has been impacted by the global IT outage and where?
A spokesman 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.
"We're urgently raising this issue with the NHS England."
The Wilmslow Health Centre in Cheshire wrote on X that practices "using the NHS commissioned GP computer system EMIS are currently without access to their IT systems".
It added: "This is beyond the control of GP surgeries. Please bear with us until we have our IT systems back online."
Solihull Healthcare Partnership in the West Midlands said on X: "Unfortunately there is a national issue with EMIS Web - our clinical computer system.
"This will affect our ability to book/consult with patients this morning.
"We will update patients when we can. We apologise for the disruption."
Windrush Medical Practice in Witney, Oxfordshire, said it is continuing as normal but urged patients with "routine concerns" to wait until Monday.
Central Lakes Medical Group in Ambleside wrote: "We're impacted by the IT outage.
"This will have a big effect on us, so apologies in advance for the inconvenience caused, and delays on the phone."
Another post by Pocklington Group Practice in the East Riding of Yorkshire said: "Due to ongoing Windows issues affecting IT worldwide, the practice is currently unable to function as normal.
"This may result in appointments needing to be cancelled and rearranged. Updates will follow when available."
Salisbury District Hospital also confirmed in a post on social media that it had been impacted.
It said: "We are suffering some delays at our hospital with our administrative services due to the global IT outage.
"We ask patients and visitors to please bear with us as we use alternative methods."
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