Sunderland have apologised for a "serious error in judgement" after a bar at the Stadium of Light was redecorated with Newcastle slogans.
The Black Cats Bar, to be used by away fans, was furnished with the signage for Saturday's FA Cup third-round tie.
Pictures, including a sign emblazoned with "keep the black and white flying high", had circulated prior to the Tyne-Wear derby taking place.
In a statement, Sunderland said the bar would "return to its original state".
"Sunderland AFC acknowledges that a serious error in judgement was made in relation to Black Cats Bar earlier this afternoon," the Championship club said.