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. Pharmacy services, such as access to prescriptions, have also been affected.
The outage has led to long queues at UK airports and has knocked some TV channels off air.
Cyber security firm Crowdstrike said the global IT issues were caused by a defect in a content update and was not a security incident or cyberattack.
“The issue has been identified, isolated and a fix has been deployed,” said George Kurtz, CEO of Crowdstrike.
Pat McFadden, Chancellor of the Duchy of Lancaster, said government teams were coordinating their response to the major outage through Cobra – the emergency response committee.
“Ministers are working with their sectors and respective industries on the issue,” he wrote on X, formerly Twitter.
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.”
Professor Kamila Hawthorne, chair of the Royal College of GPs, said the outage was of “serious concern” asked people to be “bear with us” as clinics worked to keep services going.
Long queues have been reported at airports, and British Airways says some of 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.”
Some flights into the UK have also been disrupted. Jemma Wheeler, 30, said her family of five was stuck at Palma de Mallorca Airport after the IT issues caused delays at check-in.
“We thought it was a quick, two-hour flight and we would be back in no time to enjoy the nice weather at home,” she told the BBC.
Those waiting for their flights are being urged to keep an eye on communications from their airlines and to keep essential items in their carry-on if they check in bags.
Train services are still running, although the National Rail Enquiries website has warned of “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 see alterations and cancellations.
British TV channels Sky News and CBBC are now both back on air after having been affected.