NHS services across England have been hit by IT failure, believed to be caused by a large-scale cyber-attack.
Trusts and hospitals in London, Blackburn, Nottingham, Cumbria and Hertfordshire have been affected.
Some GP surgeries have had to shut down phone and IT systems while A&Es have told people not to attend unless it's a real emergency.
NHS England says it is aware of the issue and is looking into it.
Among those affected is the East and North Hertfordshire NHS Trust which says it is experiencing problems with computers and phone systems.
It has postponed all non-urgent activity and is asking people not to come to A&E at the Lister Hospital in Stevenage.
IT specialists are working to resolve the problem as quickly as possible, a statement from the trust says.
Also affected is Derbyshire Community Health Services NHS Trust which says it has shut down all of its IT systems following a "secure system attack".
A GP from a surgery in York said: "We received a call from York CCG [Clinical Commissioning Group] around an hour ago telling us to switch off all of our computers immediately.
"We have since remained open, and are dealing with things that can be dealt with in the meanwhile."
Meanwhile, Blackpool Hospitals NHS Trust has asked people not to attend A&E unless it was an emergency because of computer issues.
But the NHS in Wales said it had a separate IT system and had not been affected by the cyber attack.