The NHS has been hit by a major cyber attack, with hackers demanding a ransom.
Hospitals are understood to have lost the use of phonelines and computers, with some diverting all but emergency patients elsewhere.
At Lister Hospital in Stevenage, the telephone and computer system has been fully disabled in an attempt to fend off the attack.
Patients have tbeen told not to come to A&E and all non-urgent appointments and operations have been cancelled.
East and North Hertfordshire NHS trust said in a statement: “Today the trust has experienced a major IT problem, believed to be caused by a cyber attack.
“The trust is postponing all non-urgent activity for today and is asking people not to come to A&E – please ring NHS111 for urgent medical advice or 999 if it is a life-threatening emergency.
“To ensure that all back-up processes and procedures were put in place quickly, the trust declared a major internal incident to make sure that patients already in the trust’s hospitals continued to receive the care they need.”
Health officials are understood to have declared a major incident and ordered a meeting of national resilience teams.
NHS Digital said: “We’re aware that a number of trusts that have reported potential issues to the CareCERT team. We believe it to be ransomware.”
There are reports that trusts affected include East and North Hertfordshire, North Cumbria, Morecambe Bay hospitals, Blackpool, and Barts Health in London.
A number of GP surgeries also say they are also unable to use their systems.
They said trusts had their computer systems almost entirely shut down.
Services affected are thought to include picture archiving communication systems for x-ray images, pathology test results, phone and bleep systems and patient administration systems.
The source added: “This will mean delays and a focus on the sickest patients. I’ve seen it once before and we relied on local trusts supporting each other. If truly widespread then that’ll not be an option.”