Seven hospital trusts are still turning away patients from its Accident and Emergency departments after Friday's cyberattack, NHS England sources say, according to Sky News.
While patients with appointments on Monday (15 May) have been told to turn up as usual unless they have been told to stay away, there are now growing concerns that further cases of ransomware may crop up on Monday.
Barts Health NHS Trust, the largest in the country, was still facing IT disruptions, that is leading to delays and cancellations in patient appointments. It is advising the public to use other NHS services where possible.
In a statement posted on its website on Sunday (14 May), it said that although all of its hospitals remain open for emergency care, some ambulances are diverted to neighbouring hospitals.
"We have slower than usual access to pathology and diagnostic services which means our service are running slower than normal, but they remain safe."
It also said that it is cutting the volume of planned services on Monday "to ensure we can continue to run services safely."
Trusts in Lincolnshire and Southport and Ormskirk, in the north-west have cancelled all routine appointments on Monday while staff have been warned to be extremely careful when logging into the NHS network, the TV channel reports.
United Lincolnshire Hospitals NHS Trust said that outpatient appointments, diagnostic tests and routine operations will also be cancelled as work continues to restore its IT systems.
United Lincolnshire Trust and Northumbria Healthcare NHS Trust took to social media to warn its staff that they should not switch on any devices unless specific clearance has been given by their managers.
"All staff should report for duty tomorrow as usual - however please DO NOT switch on or log into any laptops, mobile devices or desktop PCs until advised to do so by your line manager," the Northumbria Healthcare warned.
Patients told to steer clear of GPs
Separately, the head of UK's family doctors has urged patients without appointments to steer clear of their GPs at the start of the week unless it is a matter of urgency.
Dr Helen Stokes-Lampard, the chairman of the Royal College of GPs said: "The concern is that on Monday morning the appointment system may not be working, some places may not be able to access routine results, even the phone lines in some cases may not be working.
"If it is routine we are saying please leave it a day or two - if you are urgent we will prioritise but if not please give us a couple of days," she said, reports The Telegraph.
The true extent of the damage caused by the cyberattack will only be known when doctors across the country try to log on to their systems.
"In some cases, computer systems will have been patched over the weekend but what we are frightened about is everyone logging on and there being a massive drain on the system as every computer is updated," she added.
Further cases of ransomware may come to light
The National Cyber Security Centre said: "That, as a new working week begins it is likely, in the UK and elsewhere that further cases of ransomware may come to light, possibly at a significant scale."
In its latest update, the NCSC warned that although there has not been new attacks, there still could be machines and networks that have already been compromised but have yet to be detected.
And there is also the danger that existing infections from the malware can spread within networks," it said.