A hospital trust in Yorkshire is among a series across the country that appear to have been targeted by a suspected attempted cyber attack.
NHS trust IT systems in London, Blackburn, Nottingham, Cumbria and Hertfordshire have also reportedly been affected.
York Teaching Hospital NHS Foundation Trust, which runs hospitals in York and Scarborough, confirmed it was among those experiencing IT difficulties.
The suspected cyber attack also appears to be affecting some Clinical Commisioning Groups (CCGs), including NHS Sheffield CGG.
The CCG said on Twitter: "We're aware of an IT issue affecting NHS computer systems. Please do not attend A&E unless it's an emergency. Thank you for your patience."
Leeds Teaching Hospitals NHS Trust and Mid Yorkshire Hospitals NHS Trust, which runs hospitals in Wakefield and Dewsbury, said they were not currently affected.
A spokesman for NHS England said there was "an issue with IT".
One pharmacist in Yorkshire said on Twitter that he had been forced to hand-write notes while seeing patients without full histories after a GP practice was experiencing IT issues.
Blackpool Teaching Hospitals NHS Trust and the resort town's CCG has warned of problems.
The CCG tweeted: "We are aware of an IT issue affecting some GP computer systems.
"Patients are asked for understanding whilst the issue is resolved.
"Please avoid contacting your GP practice unless absolutely necessary. Should you wish to obtain non-urgent medical advice, please call 111.
"Please also only attend the Walk-In Centre and A&E department if absolutely necessary."
It comes months after Barts Health Trust, the largest NHS trust in England, was hit by a ransomware cyber attack.
The trust sent a message to staff urging them not to open email attachments from unknown senders.
Viruses such as ransomware are normally delivered via emails which trick the recipient into opening attachments and releasing malware onto their system in a technique known as phishing.