Hundreds of planned operations and outpatient appointments have been cancelled across Lincolnshire after an NHS computer network was attacked.
Northern Lincolnshire and Goole NHS Foundation Trust (NLAG) said systems were infected with a virus on Sunday, with it treated as a "major incident".
The trust, which runs hospitals in Goole, Grimsby and Scunthorpe, said the measures would remain into Tuesday.
United Lincolnshire Hospitals NHS Trust (ULHT) also had to cancel operations.
Dr Karen Dunderdale, NLAG deputy chief executive, said: "A virus infected our electronic systems yesterday, and we have taken the decision, following expert advice, to shut down the majority of our systems so we can isolate and destroy it.
"Our main priority is patient safety. All adult patients should presume their appointment/procedure has been cancelled unless they are contacted. Those who turn up will be turned away."
The trust added that inpatients would be cared for and discharged as soon as they were medically fit, with major trauma cases and high risk women in labour being diverted to neighbouring hospitals.
It said: "We are reviewing the situation on an hourly basis. Our clinicians will continue to see, treat and operate on those patients who would be at significant clinical risk should their treatment be delayed."
Further updates will be posted on the trust website.
ULHT shares four of its clinical IT systems and said it had to cancel operations "unless there is a clinical reason not to".
Mark Brassington, chief operating officer at ULHT, said: "We have a plan in place to minimise risks to patients which includes reverting to manual systems.
"The biggest impact on the trust is in processing of blood tests, access to historical test results and availability of blood for blood transfusions.
"Our number one priority is keeping patients safe so we are cancelling all planned operations tomorrow unless there is a clinical reason not to."