Tens of thousands of people awoke on Christmas morning to a surprise from an unexpected benefactor - Santander.The bank mistakenly deposited £130m into 75,000 accounts on 25 December.
Santander's staff are now rushing to claw back the money, although the job is being made more difficult because much of it was deposited in accounts at rival banks, according to The Times.
The error occurred when payments from 2,000 business accounts were made twice.
"We're sorry that due to a technical issue, some payments from our corporate clients were incorrectly duplicated on the recipients' accounts," the bank said in a statement.
"None of our clients were at any point left out of pocket as a result and we will be working hard with many banks across the UK to recover the duplicated transactions over the coming days."
It said the mistake may have meant that some people were, in effect, paid twice from their employer's account, although the second payment was funded by Santander.
The bank stressed that it had already begun speaking to the rival banks - which The Times said included Barclays, HSBC, NatWest, Co-operative Bank and Virgin Money.
Santander said those banks would "look to recover the money from their customers' accounts."
However, it was not clear how the banks would respond if their customers had already spent the money, meaning returning it would push them into overdraft.
Santander indicated that it may contact people directly to get the money back.