A cyber attack against HSBC has caused disruption for customers trying to log in on a key day for managing their money.
The bank said it successfully defended itself against a "denial of service attack" on Friday - but many customers still struggled to log on to its online and mobile services.
HSBC said its services had partially recovered - but it was continuing to experience attempted attacks and it was closely monitoring the situation with the authorities.
The last Friday of the month is payday for many people, and taxpayers are also trying to get their finances in order ahead of the self-assessment deadline on Sunday, when tax returns must be returned and any tax owed must be paid.
HSBC pledged to waive any fees racked up by customers as a direct result of the latest incident and said it will help customers with any issues that have arisen.
Frustrated HSBC customers pointed out that it is not the first time this month they have been blocked from accessing their accounts online.
HSBC apologised to customers earlier in January after it suffered online banking glitches lasting for two days running.
Those problems in early January were due to a "complex technical issue" with its internet banking systems and not a cyber attack or any other malicious act, HSBC said.
This time around, HSBC said it had been the subject of a cyber attack - but customers' personal details have not been compromised.
A denial of service attack happens when hackers attempt to prevent people from using a service by overloading it with huge numbers of requests and web traffic, forcing the system to crash.
John Hackett, HSBC's UK chief operating officer, said: "HSBC's internet and mobile services have partially recovered, and we continue to work to restore a full service.
"We are continuing to experience attempted denial of service attacks and we are closely monitoring the situation with the authorities.
"We apologise for the disruption and inconvenience this may have caused.
"Customers who are unable to access the service and need to make an urgent payment can visit our branch network or contact Personal Telephone Banking between 8am and 10pm on 03457 404 404. Please be aware that we are experiencing extremely high call volumes."
HSBC, which has 17 million personal and business banking customers across the UK, has not said how many people have been affected.
The bank's website has been displaying an error message, saying: "Sorry, there appears to be a system problem. Please try again later."
One Twitter user said: "Cannot believe @HSBC online banking is STILL down ... seriously?!"
Another said: "So #HSBC online banking is down ... again! Guess I should just re-read the apologetic letter I rec'd only last week re: the previous outage!"
Someone else complained on Twitter that customer services had earlier appeared not to know about the cyber attack, saying they were asking people having problems logging in to "clear their caches and cookies".
The issues with HSBC follow the high-profile cyber attack last year involving telecoms giant TalkTalk, which resulted in customers' personal details being accessed.