Brits needing to phone the NHS 111 service have been warned of delays from a "major" computer outage due to a cyber attack that has reportedly left staff using pen and paper.
A crucial system used by NHS 111 workers across the country has been shut down by hackers feared to be linked to a hostile state.
Referring patients for care are carried out on the system including ambulances being dispatched, out-of-hours appointment bookings and emergency prescriptions.
The "security issue" was identified at about 7am on Thursday, software and services provider Advanced said, and indicated it might not be fully resolved until next week.
Potential state-sponsored cyber attacks from Russia have been warned of by the Five Eyes international intelligence alliance that includes the UK, Australia, Canada, New Zealand and the United States.
Intelligence suggests that Russia has been looking to carry out cyber attacks in response to economic sanctions imposed following the invasion of Ukraine.
Key UK sectors including the NHS have been warned of the threat and to have back-ups in place.
But the outage has seen NHS 111 staff "working on paper" and it is "negatively affecting" response times, according to a letter from NHS bosses sent to London GPs, reported the Telegraph.
MP Bob Seely on the foreign affairs committee, reportedly said: "It is undoubtedly true that cyber warfare is one of the tools of modern hybrid, full-spectrum conflict. It is used by adversarial states, including Russia, and other states like China.
"This attack could be criminal gangs acting with the tacit support of the Russian state or it could be the Russian state itself. Considering that we are one of the major supporters of Ukraine, if it is the Russians it's not exactly going to be unexpected."
The Welsh Ambulance Service described the outage as "significant", "major" and "far reaching" and said it was affecting all four nations of the UK.
The ambulance service said partners across Wales have "developed and deployed plans so services can continue to operate".
It warned the weekend will be a busier time than usual for NHS 111 Wales and said while capacity to answer calls is being "maximised" by the ambulance service and local health boards, "it may take longer for calls to be answered and we thank the public for their patience".
An NHS England spokeswoman said NHS 111 services are still available and that there is "currently minimal disruption".
They said "tried and tested contingency plans are in place for local areas who use this service".
A spokesman for the Scottish Government said it was aware of reported disruption to one of NHS Scotland's IT suppliers' systems and is "working with all health boards collaboratively on a four nations basis with the National Cyber Security Centre and the supplier to fully understand potential impact".
It said "continuity plans" are in place.
A spokesman for Northern Ireland's Department of Health said they are working to keep disruption to a minimum.
He said: "As a precaution, to avoid risk to other critical systems and services, access to the company's services from the HSC (Health and Social Care system) has been disabled, while the incident is contained.
"Business contingency measures have been instigated for the affected HSC organisations and areas."
Simon Short, chief operating officer at Advanced, confirmed the incident was related to a cyber attack and said it had taken action which contained the attack, adding that "no further issues have been detected".
He said: "Early intervention from our incident response team contained this issue to a small number of servers representing 2% of our health and care infrastructure. The protection of services and data is paramount in the actions we have and are taking.
"We continue to work with the NHS and health and care bodies as well as our technology and security partners, focused on recovery of all systems over the weekend and during the early part of next week.
"In the meantime those NHS impacted services will continue to operate [using contingency]."
