Outages have been reported today across airlines, supermarkets, banking and communication services as well as the NHS and trains.
Crowdstrike, which provides cyber attack monitoring and protection to many major businesses, said the problem was caused by a "defect found in a single content update for Windows hosts", adding it was not a security incident or cyber attack.
Latest on worldwide IT outage
Sky News has been tracking issues like flight delays and cancellations, reports of service outages and Google searches to see when people first started noticing problems.
Airlines
The first signs of trouble in the UK were from users of airline services across Ryanair, British Airways and WizzAir.
Air travellers are often up early to get to the airport and problems with checking in can cause additional stress and panic. Reports first started at around 7am before peaking at around 8.30am.
Ryanair has had the highest number of outage reports according to Downdetector, a website which records issues people have raised with IT services and connectivity.
As of around 10.30am, more than nine in 10 flights from some major UK airports were either delayed or cancelled.
Passengers at airports across Europe have been affected too, with Amsterdam Schipol, Barcelona, Madrid, Rome, Stuttgart and Naples all reporting major disruptions.
About half of the Ryanair issues were related to check-in, with the remainder split between website and app problems.
More than three-quarters of British Airways outage reports were connected to check-in, while more people were having issues with the Wizz Air app.
Banking
Reports about banking services, like Visa and Mastercard, and individual banks such as Nationwide and Lloyds, also started coming in early. They started around 7am but peaked a bit later at 8am.
Visa appears to have been worst affected, with issues evenly split between purchases and payments.
Nationwide has also seen issues related to online banking specifically, with two-thirds of reports citing this issue.
Supermarkets
The first issues from supermarkets started coming in at around 9am, peaking at about 10am.
There have been fewer reports from the likes of Waitrose and Lidl compared with Tesco and Sainsbury's, but that doesn't mean they've been less badly affected.
It could just be a reflection of the fact that more people are trying to interact with services at Britain's two biggest supermarkets.
Most of the reports - about three quarters in total across all supermarkets - were related to checkout, with website issues and order tracking responsible for the remainder.
There have been fewer reports so far about any issues relating to supermarket app services, but again this could be a reflection of which services people use most often rather than them being immune from issues.
Communications
Microsoft Teams and Microsoft 365 have been particularly badly affected - there was a rush of reports at 8am as people tried to log in to check emails or messages.
There also appeared to have been a smaller rise in reported issues with other communications channels including WhatsApp and O2.
Most problems reported with WhatsApp related to sending messages, and two-thirds of issues reported with O2 were related to lack of signal, with a further 21% to do with mobile internet.
The majority of problem reports for Microsoft 365 were related to issues with server connection, while a quarter were to do with login issues and a fifth related to OneDrive.
The way many people first noticed issues with their Microsoft products was by seeing a "blue screen of death" on their computer monitors.
Google searches for the term "blue screen" have reached their highest level in five years today.
Searches worldwide peaked at 7.15am UK time, with people in Australia, China, India and North America all looking up the term.
Singapore, Hong Kong and Australia are the three territories with the most relative search interest in the past 24 hours, but that may be because they were more likely to have been awake when the problems first started.
We will update this story later on to see which services have recovered and which are having lasting problems.
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