A Welsh student says she quit her job at a fast food restaurant - claiming she was told to speak only English to customers.
Ceri Hughes, from Felinheli, near Caernarfon, started work at KFC in Bangor High Street at the end of June and was trained to use the till and how to handle customer orders.
The Bangor University history student said: "The supervisor told me I had to take every order in English and to speak in English with customers.
"I carried on speaking Welsh with customers who spoke Welsh to me and the same supervisor reminded me to speak English with customers.
"It is not acceptable I should be told to speak English only with customers."
The 27-year-old subsequently quit the part-time job and is now looking for more part-time work over the summer before resuming her university studies.
Ms Hughes said she was considering making a complaint about the fast food giant to the Welsh language Commissioner.
But KFC says it wants staff and customers to speak whatever language they feel comfortable with.
A spokesman for Cymdeithas yr Iaith, the Welsh Language Society, said it was "a very serious allegation".
Tamsin Davies said: "If true, it’s completely unacceptable. KFC should apologise immediately and adopt a clear policy that staff and customers have an unconditional right to communicate in Welsh.
"The Welsh Language Measure 2011 makes it illegal for people or organisations to interfere with the freedom of people in Wales to use the Welsh language. We have contacted the Welsh Language Commissioner’s office asking him to open an inquiry into these allegations.
“On the ground, over recent months, we have seen a concerning rise in allegations of employers banning the use of the language. It's important that people complain to the Welsh Language Commissioner directly about any such incidents.
"We have also encouraged the Commissioner and others to do far more to raise awareness of the law and to tackle the prejudice against minorities and the Welsh language which lies behind these terrible incidents and policies.”