A Greggs bakery worker has been suspended after he allegedly told a customer who ordered in Welsh the language sounded "like Tourette's".
Sioned Howells, 18, reported the incident in Lampeter, Ceredigion, on her Twitter account.
She tweeted the employee said: "That was Welsh?? Sounded more like you had tourettes to me."
Greggs said it was "deeply sorry" for any offence caused and was investigating further.
The company said in a statement: "We take this matter very seriously and the member of staff has been suspended whilst we investigate further.
"This incident goes against our values and should never have happened. We are deeply sorry for any offence caused."
Osian Rhys, vice chairman of the Welsh language campaign group, Cymdeithas yr Iaith, said the incident "is terrible if true".
"It's happened, not just because of the attitude of one member of staff, but also partly because the language legislation doesn't cover private businesses," he said.
The group has written to the chief executive of Greggs asking for a meeting to discuss Welsh language policy.
It is also calling on the government to impose language duties on high street stores so "the use of Welsh is normalised in all parts of life".
Nes i archebu yn Gymraeg a'r ateb ges i odd - "That was Welsh?? Sounded more like you had tourettes to me" yn Greggs LLAMBED😤— sioned (@sionedhowells_) November 4, 2017