Many online customers of HSBC were unable to access their internet and mobile banking services early on Friday morning - instead, they received a message telling them to "try again later".
By mid-morning, the bank said services had been restored, but some customers - taking to social media to air their displeasure - disputed this.
HSBC did not give a reason for the outage and could not immediately identify how many customers had been affected, but it apologised for the glitch, saying: "We are sorry for any inconvenience caused".
Outages of this kind are not uncommon and earlier this year, some NatWest customers were horrified to discover that cash had appeared to disappear when they attempted transfers.
Lloyds, Halifax and RBS customers have also suffered glitches to online services this year.