US company says the issue is “not a security incident or cyberattack” but is a “defect” in a “single content update for Windows hosts”.

The source of the global IT outage has been “identified” and a “fix deployed” but it “could take some time”, the head of US cybersecurity company CrowdStrike has said.

It was caused by a “bug that was related to our update”, George Kurtz said.

While some systems are now back online, others “automatically won’t recover” he admitted.

Banks, airlines, train companies, telecommunications companies, broadcasters and supermarkets have been affected.

There was a “weird interaction” that did not appear to happen on every Windows system, Mr Kurtz said, and his company is trying to isolate where that “negative interaction was”.

In a note of reassurance, he said the issue was “not a security incident or cyberattack”.

“Mac and Linux hosts are not impacted,” he added.

“The issue has been identified, isolated and a fix has been deployed.”

Microsoft IT outage: Follow live

“We’re deeply sorry for the impact we’ve caused to customers, to travellers, to anyone affected by this including our [client] companies,” Mr Kurtz told Sky News’s partner network, NBC.

“We know what the issue is and have resolved [it] now – essentially the system was sent an update and that update had a software bug in it.”

His team is looking at the “safety, security and quality” of the firm’s updates and will “have to go back and see what happened”.

Asked how such an event could take place, Mr Kurtz said: “When you look at the complexity of cyber security, you’re always trying to stay one step ahead of the adversaries.

“Our systems are always looking for the latest attacks from these adversaries that are out there so this content update went out as it’s been doing for many, many years.”

CrowdStrike will provide “complete and continuous updates” on its website, Mr Kurtz said, referring customers to the “support portal for the latest updates”.

The company’s “Falcon Sensor” software was allegedly causing Windows to crash and display a blue screen, according to an alert sent by the company to its clients and seen by the Reuters news agency.

The alert reportedly included a manual workaround to rectify the issue.

Microsoft said a resolution for Windows devices is “forthcoming”.

“We are aware of an issue affecting Windows devices due to an update from a third party software platform,” the IT giant said.

“We anticipate a resolution is forthcoming.”

The IT outage is “causing disruption in the majority of GP practices” in England, NHS England has said.

There is currently no known impact on 999 or emergency services, a statement read.

Airlines around the world are issuing handwritten tickets as the fallout continues.

Ryanair were handing out such tickets at Stansted Airport after having problems printing.

Read more:
IT outage: What has been affected and where?

Louise Haigh, the transport secretary, said the government is working “at pace with industry” to address IT issues causing train and air disruption.

She added on social media: “There are no known security issues at present.”

Air traffic control has not been affected.

A spokesperson for National Air Traffic Services said: “We have no IT issues and are operating normally.”

Belfast Airport, however, has been using whiteboards instead of electronic screens for customer information.

CrowdStrike was founded in 2011 to “fix a fundamental problem”, it says on its website: “Sophisticated attacks that were forcing the world’s leading businesses into the headlines.”

Its approach combines the “most advanced endpoint protection with expert intelligence to pinpoint the adversaries perpetrating the attacks”, the website says.