The BBC has apologised after an off-screen voice turned the air blue on BBC1's Breakfast show.

Viewers heard a disembodied voice exclaim "What the f*** is that?" at the start of an item during the TV programme this morning.

Presenter Susanna Reid was introducing conductor John Wilson as a guest on the show when the gaffe occurred.

The foul-mouthed outburst became a popular talking point online as users sent messages about the slip on Twitter.

The BBC said a microphone had been left on outside the studio which picked up the four-letter word.

A spokeswoman said: "During the programme this morning, we accidentally played out not-for-broadcast audio containing an audible swear word. The error was caused by a radio microphone inadvertently being left on outside our studio.

"Our presenters apologised on air as soon as the mistake was identified and we are reviewing our procedures accordingly."

The show has previously had to apologise for broadcasting swear words when an uncensored tape of actor Christian Bale letting fly with a torrent of abuse was mistakenly aired.

Dame Helen Mirren and film director Nicolas Winding Refn have also used bad language on the programme.