International efforts are under way to track down the perpetrators of the unprecedented cyber attack that threw a large part of the NHS into chaos and affected business in countries around the world.

Europol said its cybercrime specialists will support affected countries as a "complex international investigation" to identify the culprits begins, as security experts warned that another major attack could happen soon.

Oliver Gower, of the National Crime Agency, said: "Cyber criminals may believe they are anonymous but we will use all the tools at our disposal to bring them to justice."

Meanwhile a British cyber security researcher, known online as MalwareTech, told the BBC there could be another attack coming "quite likely on Monday".

MalwareTech was hailed as an "accidental hero" after he helped prevent the virus from spreading further when he and another cyber security researcher stumbled upon a "kill switch" in the malware code.

The 22-year-old said: "We have stopped this one, but there will be another one coming and it will not be stoppable by us.

"There's a lot of money in this. There's no reason for them to stop. It's not really much effort for them to change the code and then start over.

The NHS systems are believed to have been hit by a ransomware cyber attack using malware called "Wanna Decryptor".

Forty-five NHS division were affected by the attack. Operations and appointments were cancelled for patients across the country as 48 trusts in England and 13 health boards in Scotland were attacked by the ransomware.

Medical staff reported seeing computers go down "one by one" as the attack took hold, locking machines and demanding money to release the data.

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.

On Saturday a former head of the body managing the NHS' cyber security system said an online attack on the health service was "always going to happen".

He said although the government has invested in protecting against a cyber attack, it can be "difficult" to ensure trusts spend money on cyber security.

Speaking after a Cobra meeting on Saturday, Home Secretary Amber Rudd admitted "there's always more" that can be done to protect against cyber attacks.

But she stressed that attack had affected organisations in scores of countries.

"If you look at who's been impacted by this virus, it's a huge variety across different industries and across international governments. This is a virus that attacked Windows platforms. The fact is the NHS has fallen victim to this," she said.

"I don't think it's to do with that preparedness. There's always more we can all do to make sure we're secure against viruses, but I think there have already been good preparations in place by the NHS to make sure they were ready for this sort of attack."

NHS Digital, which manages the health service cyber security, said fewer than 5 percent of devices within the health service still use the old Windows XP system.

Nissan UK confirmed it was affected, but said there had been "no major impact".

It is understood its plant in Sunderland is not due to have another production shift until Sunday night.

A spokesman said: "Like many organisations around the world, some Nissan entities were recently targeted by a ransomware attack.

"Our teams are responding accordingly and there has been no major impact on our business. We are continuing to monitor the situation."