Miriam Margolyes swore during a chat with Robert Peston long before the watershed last month.
The live show was thrown into turmoil after the 76-year-old decided it was time for one of her very saucy tales.
Miriam began to tell Robert, 57, about one particular anecdote involving Warren Beatty while filming his movie, Reds.
However, Robert was forced to apologise, sparking an Ofcom investigation for broken broadcasting rules.
Miriam had warned: “I’m going to use a bad word, can I use that in the morning on a Sunday?”
Robert seemed oblivious to what was coming and agreed that she could continue.
Miriam went on: “Well when I went for an interview for Reds a long time ago, I was told I could only see him in his trailer.
“So I knocked at the door, in the lunch hour, and he said, ‘Come in’. I opened the door, he looked me up down, up and said, ‘Do you f**k?’.”
Robert was completely flabbergasted by Miriam’s use of the explicit term and instantly started to giggle at the live TV blunder.
“Apparently we can’t say that – I’ve just been told,” he defended.
Robert also quipped: “You’ve just got me sacked, which was a public service.”
Miriam seemed unaware of the extent of her blunder and merely continued with the story.
“Oh well,” she sighed. “I said [to Warren],‘Yes, but not you’.”
Studio guests included MP Sarah Wollaston and Labour peer Lord Adonis who found the blunder hilarious.
Ofcom said: “We are investigating whether this programme broke our rules on offensive language before the watershed.”
Peston On Sunday airs weekly on ITV at 10am.