The source of the global IT outage has been "identified" and a "fix deployed", the head of US cybersecurity company CrowdStrike has said.
Banks, airlines, train companies, telecommunications companies, broadcasters and supermarkets have been affected.
CrowdStrike CEO George Kurtz said the issue is "not a security incident or cyberattack" but is a "defect" in a "single content update for Windows hosts".
Microsoft IT outage: Follow live
"Mac and Linux hosts are not impacted," Mr Kurtz said.
"The issue has been identified, isolated and a fix has been deployed."
CrowdStrike will provide "complete and continuous updates" on its website, Mr Kurtz added, referring customers to the "support portal for the latest updates".
CrowdStrike's "Falcon Sensor" software is 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, issued at 0530 GMT on Friday, 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.
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