HSBC says it has "successfully defended" itself from a cyber-attack on its online banking systems.
However, customers are angry because some parts of its online service were down for much of the day. Today is pay day for many people, with bills due to leave their accounts and the self assessment tax return deadline is just two days away.
HSBC has apologised for the disruption and stressed that it is already working with authorities to "pursue the criminals responsible".
On the subject of self assessment tax reutrns, a spokesperson for HMRC said: "Where taxpayers need information from their HSBC account, and they are currently unable to access this they can include an estimate in their return in order to file by 31 January. They have 12 months from the date they file to amend this with the correct information."
HSBC stressed that no customer data was stolen in the attack.
A spokesperson said: "HSBC internet banking came under a denial of service attack this morning, which affected personal banking websites in the UK.
"HSBC has successfully defended against the attack, and customer transactions were not affected. We are working hard to restore services, and normal service is now being resumed. We apologise for any inconvenience this incident may have caused."