Britain's biggest bank, HSBC, is still trying to repair its online banking system, which has failed for the second day running.
The personal banking website is not allowing customers to log on, while business accounts are running slowly.
On Tuesday afternoon, HSBC said it was getting closer to solving the problem, which affected online accounts for up to nine hours on Monday.
A spokesperson said it was "a complex technical issue within our systems".
The bank is running tests across its servers to try and identify the exact problem, and said that its IT team had been working "non-stop" since yesterday to find a solution.
HSBC said it was not a distributed denial of service (DDOS) attack and was not caused by anything external.
"This is not a cyber-attack or any other malicious act," said John Hackett, HSBC's UK chief operating officer.
The bank said its mobile app for personal customers was still functioning.
In total, HSBC has 17 million account-holders in the UK.
Business customers were experiencing "significantly reduced capacity", with all services running much more slowly than normal, the bank said.
Customers of HSBC's subsidiary, First Direct, are not affected.
HSBC said that no customers would lose out as a result of the issue.
"Any fees customers incur as a result of this outage period will be waived," said Mr Hackett.
Nevertheless, angry customers took to Twitter to complain. One described the situation as "shocking disgraceful customer service".
Others said they had been charged a £5 daily overdraft fee, but were unable to put money into their accounts.
Sam Middleton-Bray, from Hampshire, said she had had difficulty getting information about what was happening.
"It's incredibly frustrating and we're not getting a lot of information from HSBC other than their brief messages on Twitter. Their website says to call the 0345 number, but I called it yesterday three times and got an engaged tone," she told the BBC.
One or two customers complained that the mobile app was not working either, despite HSBC's assurances.
HSBC was hit by a previous IT glitch in August 2015, which prevented 275,000 payments going through just ahead of the Bank Holiday weekend.
One senior MP called on the boss of the bank to make sure such failures are not repeated.
"The frequency of these failures across the financial services sector suggests a systemic weakness in IT infrastructure. This is concerning." said Andrew Tyrie, the head of the Treasury Select Committee.
"I will be asking the chief executive of HSBC, and the regulators, for an explanation of these failures and action taken to sort them out. They just keep coming."
HSBC customers affected by the problem are being advised to use the automated telephone banking service, on 03457 404 404. Business customers should call 03457 60 60 60.
Those with business accounts are usually restricted to transferring a maximum of £10,000 via telephone banking, but this limit has been raised to £100,000.
Otherwise, customers should go to their local branch, HSBC said.