HSBC is investigating a nationwide system failure after the bank's internet services crashed and some cash machines refused withdrawals today.
Customers who called the HSBC internet banking helpline this afternoon were also told the service was unavailable.
HSBC spokesman Mark Hemingway said: "We have identified some problems with our services. Internet banking is currently down and some of the ATMs are not working.
"We are aware of the issues and we are working to get the services up again as soon as possible."
Mr Hemingway said the problems began at 2.45pm and only affect customers in the UK.
He said he was confident there was no security risk to customers' accounts as a result of the glitch.
The problems sparked a storm of complaints on social networking site Twitter as worried customers were unable to withdraw money from HSBC cash machines and could not make transfers online.
Some customers reported First Direct services were also affected.
One tweeted: "'No food/no transport' may seem dramatic but I'm currently stuck miles from home, unable to buy tickets, nothing to eat. Not happy."
The Twitter feed from the HSBC UK press office said: "Problem is being worked on urgently. As soon as we know how long this issue will last, will let you know. Sorry - thanks for patience."
Helen Dadd, 23, an HSBC customer from Birkenhead, said: "I went out and tried three cash machines and none would let me draw my money out.
"I went to the bank and asked if it was my personal account or if it was the system and they said their system was down.
"They didn't say when it would be up. I haven't got any cash at all.
"It's just shocking."