Rage Against The Machine perform on 5 live
The BBC has apologised after Radio 5 live listeners heard Rage Against the Machine swearing in a live performance of their song Killing In The Name.
Zack de la Rocha, the US band's singer, was heard swearing four times on the station's breakfast show.
The band are hoping the 1992 hit will beat X Factor winner Joe McElderry to this year's Christmas number one.
A BBC spokesman said: "We had spoken to the band repeatedly beforehand and they had agreed not to swear."
He added: "When they did, we faded the band out and said sorry immediately.
"We apologise again to anyone who was offended."
The band were interviewed in a live link-up with the US on Thursday morning, before performing the song.
As the song was abruptly faded out during the controversial closing bars of the song, listeners heard presenter Shelagh Fogarty say, "get rid of it".
She added: "Sorry. We needed to get rid of that because that suddenly turned in to something we were not expecting.
"Well, we were expecting it and asked them not to do it and they did it anyway - so buy Joe's record."
Rage Against The Machine's '90s hit is in the running for the Christmas number one after a group was set up on website Facebook urging people to buy it.
The group was set up in protest at the dominance of the X Factor winners' singles in the Christmas charts in recent years.
De la Rocha said the current success of the song "says something about the real tensions that people are experiencing all over the UK and the United States as well".
Guitarist Tom Morello said the internet campaign "tapped into the silent majority of the people in the UK who are tired of being spoon-fed one schmaltzy ballad after another".
"We are honoured they've chosen our song to be the rebel anthem to try to topple the X Factor monopoly."
Many people queued for hours in his home town of South Shields
Some of the "windfall of proceeds" from the single would be going to British homeless charity Shelter, he said.
On Thursday afternoon, bookmakers Ladbrokes made Joe McElderry 1/4 favourite to get the number one slot on Sunday night.
It followed Ladbrokes move to make Rage Against The Machine favourites for the first time on Wednesday.
Spokesman Nick Weinberg said the number of bets taken for Rage Against The Machine had "taken us completely by surprise".
McElderry, 18, won the X Factor on Sunday night beating Olly Murs with almost two-thirds of the votes.