Willoughby, 36, fell about laughing after she made the accidental slip-up while interviewing McGuinness and Keith Lemon about their new series, The Keith and Paddy Picture Show.
“This must be your dream come true, I mean you say this seed was sown back when you were doing Comic Relief and you dressed up as movie characters,” Willoughby said to Lemon. “But this actual seed was sown in your bedroom when you were young ...”
The TV favourite quickly realised how her words had sounded and burst out laughing with her co-host Phillip Schofield as McGuinness asked: “I beg your pardon?”
Lemon added: “I was sowing seeds in my bedroom?”
Referring to an earlier segment of the programme, which saw Willoughby and Schofiled eating insects, said: “Hang on a minute - you've just been eating salty worms, now you're on about seeds in bedrooms.
“What is happening to this show?”
Lemon joked: “Yes, I was planting seeds when I was a child, in my bedroom thinking of Paddy McGuinness!”
Willoughby struggled to regain her composure as she warned Lemon: “Don't make it something it's not!
“What I was saying was, it has been a real passion project for you. It's been a dream come true. You know what I mean!”
The slip-up came moments after McGuinness and Lemon stormed onto the set and blasted Kelly Hoppen’s design segment.
Hoppen was talking Willoughby and Schofield through ornaments for the home when McGuinness took aim at the price of a bon bon holder.
He shouted: “£45 for that! I wouldn't pay £45 for that! London, London!”
A shocked Willoughby said: “Oh my gosh, what are you doing?”
This Morning is on ITV, weekdays at 10:30am.