The image appeared during the programme's news review segment when the publication of the photographs in Italy was being discussed.
The photos, which have been condemned by St James's Palace, are said to show Kate strolling on a beach on the island of Mustique with William.
Presenter Holmes said: "Earlier, during today's news review, we were discussing photographs of the Duke and Duchess of Cambridge on a beach in Mustique published in an Italian magazine.
"Unfortunately we accidentally showed an unblurred image of the magazine cover, which briefly showed the photographs. This was a deeply regrettable error and we are very sorry. We apologise unreservedly to the Duke and the Duchess."
Australian magazine Woman's Day is the second publication to use the pictures of the Duchess, who is about four months pregnant, following the Italian publication Chi.
This gaffe comes just months after This Morning presenter Phillip Schofield was roundly criticised for confronting David Cameron with a list of alleged paedophiles during the show.
ITV apologised on the next day's programme and emphasised it was not pointing the finger at anyone in particular. Schofield was forced to apologise after the list was briefly exposed on screen.