Friday’s edition of the ITV show began at 10:00 BST with Hammond and her regular co-host O’Leary at the helm, before she left at 11:30 during a segment about pets.
O’Leary, 53, addressed her absence when the show returned from a break, saying: “Welcome back to This Morning, Alison is feeling a little unwell, so she’s just had to duck out. Hopefully she’ll be back.”
He later updated viewers that she would, in fact, not be returning, “so I’ll be here for the rest of the show”.
Hammond initially rose to fame as contestant on Channel 4 reality show Big Brother before appearing on I’m A Celebrity and Strictly Come Dancing.
She became a co-host in This Morning in 2021, and was part of the team that won a National Television Award for best daytime programme the following year.
She joined Bake Off in 2023 and took over from the late Paul O’Grady on For the Love of Dogs in 2024.