Experts have been working round- the-clock to restore IT systems of Britain s National Health Service (NHS) after a large-scale cyber-hack by an international criminal gang wreaked havoc around the country.
With nearly 45 NHS organisations from London to Scotland hit in the ransomware attack on Friday, patients of the state-funded countrywide service are facing days of chaos as appointments and surgeries have been cancelled.
Ciaran Martin, who leads UK s National Cyber Security Centre, said his team was doing everything in its power to get "vital services" back up and running.
We are very aware that attacks on critical services such as the NHS have a massive impact on individuals and their families, and we are doing everything in our power to help them restore these vital services, he said.
It s important to understand that cyber attacks can be different from other forms of crime in that their sometimes highly technical and anonymous nature means it can take some time to understand how it worked, who was behind it and what the impact is, he told the BBC.
Critically ill patients were being diverted to unaffected hospitals as computer systems failed in accident and emergency (A&E) units and doctors were locked out of test results, X- rays and patient records.
NHS England said patients needing emergency treatment should go to Accidents & Emergency or access emergency services as they normally would.
However, there have been some reports of ambulances being diverted from affected hospitals. Individual NHS trusts have asked registered patients not to attend unless it is urgent.
London's Barts Health NHS Trust, the largest trust in the UK, cancelled all outpatient appointments at its five hospitals on Saturday.
The NHS has not been affected in Wales and Northern Ireland.
The assault is part of an attack that has affected organisations in more than 70 countries, including the US, China, Russia and Spain, and disrupted power and telephone companies.
Gangs are thought to have used tools stolen from US spies and dumped online by hackers linked to Russia.
This is not targeted at the NHS, it s an international attack and a number of countries and organisations have been affected," British Prime Minister Theresa May said yesterday.
The National Cyber Security Centre is working closely with NHS Digital to ensure they support the organisations concerned and that they protect patient safety. We are not aware of any evidence that patient data has been compromised," she added.
Ransomware programmes, which are sent by email and spread swiftly, can be bought on the dark web with sellers negotiating a percentage of the takings.
"We will be advising NHS trusts to modernise their platforms," UK Home Secretary Amber Rudd said.
The malware used in the NHS attack is called WannaCry and attacks Windows operating systems.
It encrypts files on a user's computer, blocking them from view, before demanding money, via an on-screen message, to access them again.
The demand is for a payment of USD 300 in virtual currency Bitcoin to unlock the files.
The virus is usually covertly installed on to computers by hiding within emails containing links, which users are tricked into opening.
A German ticket machine, a university laboratory in Italy and a number of Spanish firms - including telecoms giant Telefonica, power firm Iberdrola and utility provider Gas Natural - are among those hit by the outbreak.
Portugal Telecom, delivery company FedEx, a Swedish local authority and Megafon, the second largest mobile phone network in Russia, also said they had been affected.