The Sun published a picture of a flip chart used for staff training that had been accidentally left in the foyer of a Halifax branch in Manchester.
The chart carried the headline "We don't want", followed by bullet points:
"Hands up, we've made a mistake, we're very sorry," a spokesman for Halifax parent company HBOS told BBC News Online.
The spokesman acknowledged that it was neither right nor good business sense to exclude any type of customer.
But, he added, "it's a temporary problem that we'll fix very soon."
Angry response
The Sun's revelations provoked angry reactions from the Federation of Small Businesses.
"We are pretty annoyed with the Halifax, which purports to be a bank," a spokesman said.
"This perpetuates the myth that small firms are here today, gone tomorrow," he said.
"There has always been a view that the self-employed carry more risk, but that is where everyone starts from.
"There is a snobbish attitude in the financial fraternity."
New business
"Let's not take this out of context," said the HBOS spokesman, insisting that "we deal with people from all walks of life".
Halifax is a recent entrant to the business banking universe.
As part of its strategy following the merger with Bank of Scotland in September, Halifax introduced business banking in January in "a small number of branches in England and Wales", the spokesman said.
He acknowledged that the bank has "got a problem on occasion with heavy cash handling", but insisted that this was the bank's problem, not its customers' problem.
Business banking will be on offer from about 100 Halifax branches by the end of this year and the bank will not be excluding those listed on the flip chart, he explained.