Presenters Eammon Holmes and Ruth Langsford were reviewing the morning headlines when an uncensored shot of Italian gossip magazine Chi flashed up on screen.
A statement from ITV described the incident as "deeply regrettable", adding: "We apologise unreservedly to the Duke and Duchess."
This Morning accidentally showed an uncensored front cover of Chi magazine (pixellation added by Huffington Post UK)
The blunder comes just a day after St James's Palace released a statement saying it was "very disappointed" and that the publication of the pictures was "a clear breach of the couple's right to privacy."
The cover of Chi magazine - with the shot of four-months-pregnant Kate - has been circulating in the British media, although the Duchess's image has been blacked out.
St James's Palace had no comment about the gaffe on Wednesday morning.
It comes on the same day that Australian magazine Woman's Day announced it also plans to publish the pictures.
Eamonn Holmes and Ruth Langsford were reviewing the morning papers when the gaffe occurred
Sky News said the magazine was also expected to print photographs of Kate's sister Pippa in a bikini, and that the snaps had been taken by a member of the public, not the paparazzi.
The pictures show Kate wearing a bikini and strolling on a beach on the island of Mustique with Prince William.
In light of British privacy laws and the recent Leveson inquiry, the pictures are unlikely to be published in this country.
On a website publicising Chi - owned by former Italian prime minister Silvio Berlusconi - Kate is pictured on the magazine's front cover in a blue bikini striding knee-high through the sea above the headline "the belly grows".
A second image shows the Duke, wearing a pair of shorts, with his left arm around his wife as they walk along a beach.
The Duke and Duchess of Cambridge are holidaying in Mustique
A St James's Palace spokesman would not comment on whether William had been informed about the issue or if they were considering any form of legal action against the magazine.
The photographs were reportedly taken from a boat using a long-lens camera.
It is believed that the royal couple are still holidaying on Mustique and during their time on the island were reportedly joined by Kate's family, parents Carole and Michael Middleton and her siblings.
The publication by Chi, which went on sale on Wednesday, comes less than six months after the printing of pictures showing the Duchess topless and changing her bikini bottoms during a stay at a private estate in southern France last summer.
France's Closer magazine was the first publication to print the images in September last year and the Duke and Duchess later succeeded in getting an injunction from a French court stopping further pictures appearing in the title.
At the time Chi's editor Alfonso Signorini defended his decision to also publish the topless pictures, saying: "I am a director of a newspaper not a supermarket, I don't sell artichokes and carrots, I sell photographic scoops."