Lloyds Bank have been criticised after refusing to accept a letter written in Welsh from an Assembly Member.
Mike Hedges, the Labour AM for Swansea East, said the branch would not accept a Ucas letter in Welsh when he tried to get a student account for his daughter.
Mr Hedges said the Swansea branch had told him to "get a copy in English" and he said that was "unacceptable".
Lloyds Bank have apologised for the "error" adding staff would be made aware of correct guidance.
Welsh Language Minister Alun Davies tweeted the move was "completely unacceptable" and added, "they should treat both Welsh and English equally."
He said: "This sort of behaviour from major multinationals is wholly and completely unacceptable."
Mr Hedges told the BBC he had gone into a branch in Swansea to change his daughter's account ahead of her starting at Bangor University in September.
When the bank asked for proof she had been accepted on her course, Mr Hedges claims the Ucas letter they had, written in Welsh, was rejected.
"They just told us that it wasn't company policy and that they couldn't accept it," he said.
"It's unacceptable and wrong. If it was just a matter of not having a Welsh speaking member of staff in branch that day, or having to send it off and taking a few days, I would have understood no problem.
"But not being prepared to accept the document in any way is wrong."
'Taking immediate steps'
David Williams, spokesman for Welsh language campaigners Cymdeithas yr Iaiths said: "The incident is further evidence of the need to extend language rights to the rest of the private sector, including the banks.
"The bank's behaviour is totally unacceptable."
A spokesman for Lloyds Bank said: "We apologise for our error in this matter.
"We do recognise Ucas offers in the Welsh language and have many customer-facing colleagues who are Welsh speaking.
"We are taking immediate steps to make all our colleagues aware of the correct guidance."