The threat from the cyber attack that crippled international services "will continue to grow" as people return to work on Monday, the head of Europol warned.

Since Friday's breach more than 200,000 victims - including the NHS - across 150 countries have been infected by the Wanna Decryptor ransomware, also known as WannaCry.

Speaking to ITV's Peston on Sunday, Europol director Rob Wainwright said the attack was indiscriminate across the private and public sectors.

"At the moment we are in the face of an escalating threat, the numbers are going up, I am worried about how the numbers will continue to grow when people go to work and turn their machines on Monday morning.

"The latest count is over 200,000 victims in at least 150 countries. Many of those will be businesses including large corporations."

Organisations across the globe, including investigators from the National Crime Agency (NCA), are now working non-stop to hunt down those responsible for the ransomware.

Meanwhile health authorities are racing to upgrade security software amid fears hackers could exploit the same vulnerability with a new virus.

There have been calls for an inquiry into the circumstances surrounding Friday's major incident, with the Government and NHS chiefs facing questions over their preparedness and the robustness of vital systems.

Mr Wainwright explained: "We have been concerned for some time. The healthcare centres in many countries are particularly vulnerable. They are processing a lot of sensitive data."

A British cyber whiz was hailed an "accidental hero" after he registered a domain name that unexpectedly stopped the spread of the virus, which exploits a vulnerability in Microsoft Windows software.

NHS computer systems are expected to “return to normal” today, the Scottish Government has said.

On Friday an unprecedented ransomware attack affected thirteen health boards north of the border – including the ambulance service and NHS National Services Scotland.

Organisations in up to 150 countries were compromised by the virus which encrypts data and demands a ransom for access.

The attack affected acute hospitals in Lanarkshire, as well as GP surgeries, dental practices and other primary care centres around the country.

Justice Secretary Michael Matheson has advised people to attend appointments as he moved to reassure the public that cyber security would be stepped up.

More than 120 public bodies will be contacted to ensure their defences are adequate.

Organisations across the globe, including investigators from the National Crime Agency (NCA), are working to hunt down those responsible for the Wanna Decryptor ransomware, also known as WannaCry.

Mr Matheson said the Scottish Government was liaising closely with the National Cyber Security Centre and NHS Scotland to identify the cause of the attack.

Ministers are to convene an extraordinary meeting of the National Cyber Resilience leaders' board on Tuesday to review the response to the breach.

A "lessons learned" exercise will also take place to help mitigate the risks from further attacks.

The Justice Secretary said: "Friday's attack has highlighted the need for everyone to have appropriate and robust measures in place to protect against cyber-attacks which could strike any IT system at any time.

"NHS Scotland systems are being recovered, we expect them to have returned to normal by Monday, and it is important to emphasise that there is no evidence that patient data has been compromised.

"Patients who have appointments booked for Monday and beyond should attend as planned.

"However, we must remain particularly vigilant against further incidents and the Scottish Government is taking action to enhance security, including contacting over 120 public bodies to ensure they have appropriate defences in place.

"One of the most common methods of infecting computer systems is through links and attachments in emails.

"Therefore I would urge everyone to think twice before clicking on attachments or links from sources that they don't know.

"Police Scotland has been working closely with the National Crime Agency who are leading the UK-wide law enforcement investigative response, and this will continue.”

The attack has so far hit thousands of organisations in more than 150 countries, according to Europol, the European Union's police agency.

While the number of firms targeted reached 100,000, the number of individuals who have fallen victim to the cyber extortion attack could be much higher, said Jan Op Gen Oorth, Europol spokesman.

Other reports put that number as high as 200,000.

He said it was too early to say who is behind the onslaught and what their motivation was.

The main challenge was the fast-spreading capabilities of the malware, but he added that, so far, not many people have paid the ransoms that the virus demands.

He warned that more people may be hit by the virus today when they return to work and switch on their computers.

The National Cyber Security Centre has issued guidance to help protect systems from ransomware.

CEO Ciaran Martin said: “It is important that organisations reduce the risks of these attacks happening to them.

“There are three pieces of important advice to help protect your organisation: make sure your security software patches are up to date; make sure that you are running proper anti-virus software; back up your data somewhere else because you can’t be held to ransom if you’ve got the data somewhere else.”