Customers of Barclays Bank have been left unable to access web app and online banking, make payments to and from their accounts, or access customer services, in the wake of a significant IT outage at the bank which struck on Friday 31 January.According to Ooklas Downdetector service, reports of issues affecting UK customers began shortly after 5am on 31 January, and have remained relatively constant since then, with peaks at approximately 7am and 9:20am in the morning.In a statement posted to social media network X (formerly Twitter), Barclays confirmed it was experiencing an ongoing outage and apologised to those who have seen issues with its services as a result.Were having some problems with some of our systems today were sorry about this and were working hard to fix the issue, a spokesperson said.You may have trouble using some features in our app, Online Banking, as well as payments in and out of your accounts were also unable to help you over the telephone or our Message Us service.You can still use your cards and cash machines as normal, said the spokesperson, although this goes against reports from customer, some of whom are saying that they have had their cards repeatedly declined.The bank also said that if people have been unable to make a payment, they should not attempt to make it again. More information on affected services is available here.Banks and other financial services organisations are acutely vulnerable to IT disruption given the extent to which they are relied on by consumers and businesses alike, and such outages are not infrequent.Earlier this month, an outage at Capital One in the United States saw customers locked out of online banking services for days following a supply chain incident that was ultimately traced to a power outage at FIS Global, a third-party supplier of fintech services such as payment processing that works with Capital One.At the time of writing, there is no evidence or indication to suggest that the downtime at Barclays has been caused by a cyber attack or other form of security or supply chain incident.In instances where they are responding publicly to questions, Barclays customer service teams are currently referring only to technical issues.However, the IT outage at Barclays has a significant sting in its tail given Friday 31 January is the HMRC deadline for filing self-assessment tax returns, which must be done by midnight.This has raised justifiable concerns among self-employed customers because those who miss this deadline face an initial fixed penalty of 100 even in the event that there is no tax to pay, or payment has been made on time, with additional penalties following.People who provide HMRC with a reasonable excuse for missing the annual deadline may avoid a penalty, but all these cases are always considered on an individual basis.This has left many Barclays customers concerned, with many taking to social media to ask if the bank will compensate them for any fines incurred.Read more about IT service outagesOutages might be rare, but they are rarely cheap, any amount of downtime can cost you money. Learn how to minimise the risks and survive a cloud service outage.Datacentre generators are rarely run; however, they must go through regular testing and maintenance to ensure they work when a power outage occurs to prevent server downtime.CrowdStrike executive Adam Meyers appears before a US government committee to explain the series of errors that led directly to one of the biggest IT outages in history.