Patients in Yorkshire have been warned not to attend A&E unless it's an emergency as NHS systems appear to have been targeted by a suspected cyber attack.
York Teaching Hospital NHS Foundation Trust confirmed it was among several trusts across the country currently experiencing IT difficulties.
In Sheffield, the city's Clinical Commissioning Group (CCG) took to Twitter to warn patients about the IT issues.
In a tweet by the CCG, which has since been removed, it said: "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."
NHS trust IT systems in London, Blackburn, Nottingham, Cumbria and Hertfordshire have also reportedly been affected.
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.