A number of hospitals from 16 different NHS trusts across the UK have been hacked, according to reports.
NHS Digital confirmed the attack, and said a number of NHS organisation have reported to NHS Digital that they have been affected by a ransomware attack.
Some hospitals are having to turn patients away as they struggle to deal with the computing problems.
"The investigation is at an early stage but we believe the malware variant is Wanna Decryptor," NHS Digital said.
"At this stage we do not have any evidence that patient data has been accessed. We will continue to work with affected organisations to confirm this.
"NHS Digital is working closely with the National Cyber Security Centre, the Department of Health and NHS England to support affected organisations and to recommend appropriate mitigations."
NHS Digital added that the attack was "not specifically targeted at the NHS and is affecting organisations from across a range of sectors".
"Our focus is on supporting organisations to manage the incident swiftly and decisively, but we will continue to communicate with NHS colleagues and will share more information as it becomes available," the organisation said.
Jason Allaway, VP UK and Ireland, at digital workspace company RES, said: "It's becoming more common an occurrence to see ransomware attacks against healthcare organisations; after all, they are a prime target for attackers due to the nature of the data they hold. It's not just a monetary loss when it comes to medical facilities: it's far more important.
The fact that East and North Hertfordshire has had to suspend all of their non-urgent activity, and shut down A&E is a testament to how much they rely on their data to operate.
“Education, vigilance and proven technology such as context-aware access controls, comprehensive blacklisting and whitelisting, read-only access, automated deprovisioning and adequate back-up need to be put in place by healthcare organisations to both prevent and combat this problem as efficiently as possible," Allaway added.
"Today's events are clear evidence that many healthcare organisations still need to invest in this integrated approach to security."
East & North Hertfordshire Trust confirmed earlier that it was experiencing difficulties with its IT systems and telephone network.
We’re currently experiencing significant problems with our telephone network – which we’re trying to resolve as soon as possible.— E&N Herts Hospitals (@enherts) May 12, 2017
It is also affecting our IT systems.— E&N Herts Hospitals (@enherts) May 12, 2017
The Derbyshire NHS said on Twitter that it was aware of a major IT system attack.
We are aware of a major IT secure system attack. All IT systems have been temporarily shut down. More information will be available shortly— DCHS NHS FT (@DCHStrust) May 12, 2017
Telecom firm Telefonica, the owner of O2, has also been hacked today.
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