HSBC has banned customers from other banks from using its branches during peak hours.
The bank will refuse to serve account holders from other high street banks at its branches between 11am and 2pm.
Critics have attacked the move, saying it forces customers in rural areas where there is only an HSBC branch to bank with them.
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A spokesman for the bank said: "We exist to serve HSBC customers.
"We're happy to help and customers from other banks can come in at other times.
"It's simply that at the busiest times of the day we would prefer to deal with our own customers to ensure they aren't queuing."
The bank doubled its counter charges for banking services provided for non-HSBC customers to £5 earlier this year.
Consumer groups have attacked the bank for branch closures when the banking group posted profits of £10 billion worldwide.
"Any movement like this is the thin end of the wedge," said Eddy Weatherill, from the Independent Banking Advisory Service.
"The banking industry moves as a herd, and we could find ourselves in a situation much like with ATMs.
"It's a stealth movement to maximise profits while driving down customer service."
However, a spokeswoman for Which? said the magazine supported the move but hoped HSBC would have a "flexible" approach to rural branches.
"HSBC customers will be reassured to know they are being put first," she said. "We think that's excellent and very commendable.
"In rural branches, where people don't have any choice of banks and there isn't a post office nearby, we hope they would have a flexible policy." ..SUPL:
