The live Peston on Sunday 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.
She 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.
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’.”
Earlier this week, Miriam left another audience stunned as she took an unexpected drugs swipe at Phillip Schofield.
While filming for her new show, Miriam’s Big American Adventure, the Harry Potter star admitted that she is still friends with everyone, including the “druggies”.
She said: “I’m always scared of drugs. II haven’t taken a drug stronger than aspirin…”
Miriam then turned to Phillip, 55, and joked: “I bet you can’t say that.”
The This Morning veteran laughed off the mocking, commenting: “You say that with four seconds to spare,” after she had stayed relatively well-behaved throughout the interview.
Peston on Sunday airs weekly on ITV at 10am.