The Black Eyed Peas star previously confessed that he had to imagine his mum was in the room to help him keep his language clean.
“I just pretend my mum is in the room and for some reason the curse words stop," he said.
However the tactic didn't work at the week, after he told Cleo Higgins after her performance of Don't Let Go: “To have to go through the public and see things on iTunes — that s** — that stuff is really hard to swallow.”
He quickly added: “I’m sorry, I’m sorry, bleep it out — too late."
Producers had been drilling it into the coaches to watch their mouths this year, with editors having to cut out a lot of footage from the pre-recorded show because of the language.
“All of the coaches were warned before the live shows to be careful with their language, as there would be young children watching," a source explained. “Up to this point they have all managed to keep a lid on it, but Will got carried away with the moment on Saturday and the word just slipped out.
“But the bosses have since taken him to one side and explained that it can’t happen again."
The insider added to The Sun newspaper: “Will understands and has apologised, as he did on stage.”
The Voice UK final airs this Saturday night on BBC One.