Why have IT outages affected McDonald's, Tesco, Sainsbury's and Greggs?
Why have some retailers been plagued by technical issues?
Words by James Gray, ITV News Multimedia Producer
Greggs has become the fourth major retailer to report IT outages across its UK stores in less than a week.
Fast food chain McDonald's plus supermarkets Tesco and Sainsbury's fell foul of technical issues last weekend, with all three issuing an apology to consumers.
The frequency at which major firms have encountered IT issues has sparked debate around what exactly has caused the outages and whether there is a link.
So, why have some retailers been plagued by technical issues? ITV News explains.
Where have IT outages been reported?
McDonald's apologised to customers last Friday, after an IT system outage left customers unable to order food in its restaurants.
UK customers, along with those in Australia, New Zealand and Japan, were all briefly affected, with the company later apologising for "any inconvenience" caused.
A McDonald's spokesperson said the issue was "not related to a cybersecurity event".
The following day, two of the UK's biggest supermarkets - Tesco and Sainsbury's - issued similar statements, when they were both hit with unrelated technical issues.
Sainsbury's customers were unable to make contactless payments, following an overnight software update which left the firm unable to fulfil the "vast majority" of its online deliveries.
Meanwhile, Tesco had to cancel some online orders that were due for delivery.
Popular bakery chain Greggs became the latest company to suffer an IT outage on Wednesday, when stores were forced to close across the UK due to a "technical issue".
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Why have there been IT outages?
James Bore is the managing director of Bores Group, a chartered technology and security company.
He told ITV News that the "most likely" explanation for the IT outages was an issue occurring during a live system update.
"This is the consequence of companies developing massive, monolithic, incredibly complex systems, not managing their dependencies and not having proper quality testing before deploying live," Mr Bore said.
"That's simply what it is. We should expect more and more of this. It happens daily somewhere. It's just that these ones are close proximity and noticeable chains."
James Bore explains why several major companies have been hit by IT outages in the UK
Rumours have circulated online in recent days that the technical issues were the result of a cyber attack.
But Mr Bore rejected that suggestion, saying there is "no evidence whatsoever" of a cyber attack taking place.
He added that had such an event occurred it would have been " a lot more damaging".
Asked how often technology updates are issued, Mr Bore explained it varies, with most companies using a software development method called Agile, which uses 'Sprints' - a time-boxed period during which work is carried out to complete a system update.
Mr Bore said: "There's always faults and, again, the bigger the system, the more complex it is, the more updates you're going to get because they'll be to update just tiny fractions of that system."
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