The first minister and Arriva Trains Wales (ATW) have clashed publicly on social media over the absence of Welsh language announcements at a Cardiff station.
Carwyn Jones used his AM Twitter account to question the rail company on Saturday about Queen Street station, quipping: "Has the border moved?"
He then criticised ATW's response as "shirty" and "unprofessional".
ATW said "we sincerely apologise" for an inappropriate response to the query.
The comments from the first minister came after he had been to open the Tafwyl festival at Cardiff Castle, an event to celebrate the use of the Welsh language in the capital city.
But he turned to Twitter after being at the nearby station, stating: "English only announcements at Cardiff Queen Street today. Has the border moved?"
ATW replied an hour later to the first minister: "There has been no border moving that I have been made aware of, but I apologise if the English announcements upset you."
However, the Bridgend AM was clearly unhappy with the response.
"This not an appropriate answer. Where are the Welsh language announcements?" he said.
"A simple "sorry, we'll investigate" would have done instead of a shirty response. Unprofessional."
The comments sparked a flurry of replies across the social media network, with members of the public entering into the debate with the first minister.
Responding to the remarks on Sunday, a spokesperson for ATW issued an unequivocal 'sorry'.
"We sincerely apologise to the first minister for the inappropriate response to his query on Twitter regarding English only announcements at Cardiff Queen Street station," said the ATW official.
"Due to the high volume of rail traffic through the station, and the fact that it currently only has three platforms, destination announcements are in English only as the high number of place-name announcements would end up overlapping each other and cause confusion."
The company said generic announcements at the station were bilingual, and a Network Rail project - funded by the Welsh government - will see an additional platform at Queen Street.
"This will provide us with the opportunity to introduce bilingual destination announcements at Cardiff Queen Street," said the rail operator.
"All other staffed stations in Wales with auto announcing facilities have bilingual announcements."