The online banking system of HSBC was attacked on the morning of 29 January, the last working day of the month and just two days before the tax return deadline. It is the third time the bank's online system has gone offline within the past month.
The bank tweeted at 11.22am on 29 January, a popular payday for many, saying: "HSBC UK internet banking was attacked this morning. We successfully defended our systems... We are working hard to restore services, and normal service is now being resumed. We apologise for any inconvenience."
In a further statement, the bank added: "HSBC internet banking came under a denial of service attack this morning, which affected personal banking websites in the UK." The issue was earlier described as "a complex technical issue with out systems"/
The final Friday of the month is a common payday for many workers, and this year falls just two days before 31 January, the deadline for submitting online tax returns. This is when millions of people are expected to settle their tax bill with HM Revenue & Customs, but many HSBC customers may be currently unable to do so.
HSBC's online banking system also fell offline on 4 January, then again the following day.This was also blamed on a "complex technical issue".
'You have not successfully defended yourself'
Brian Spector, CEO of MIRACL, a data security company, said: "Not even the largest financial institutions on earth are immune from cyber attacks that disrupt business operations. HSBC is using antiquated authentication technology, what else is not up to speed such that one of the world's largest banks has been taken offline?
HSBC are claiming to have "successfully defended" the attack but if your main business is taken offline, and your website is unreachable, you have not successfully defended yourself."
The latest outage has spared fury on Twitter, as customers are unable to access their accounts and transfer money. One customer, Leanne Casey, said on Twitter: "Marvelous...today is payday...and bloody HSBC having banking issues once again."
In reply to the bank saying it "defended its systems", customer Gail Perfitt said: "You obviously need more security protection! Not really something to gloat about!"
Commonly used by hackers and online activists, a distributed denial of service (DDoS) attack is when malicious computer users bombard a website with so much traffic that its servers are unable to cope, and it falls offline.