The 51-year-old is taking a break to spend some time at home and participate in a charity cycle race in Cuba.
"I've decided to take three weeks off to be a proper mummy at home. I'm absolutely fine and dandy and there's no problem," she told replacement host Ruth Langsford on This Morning.
However, a source close to the show claimed that Britton's decision has left show bosses in a fix.
"No-one understands why Fern isn't coming back until after Christmas. It wasn't pre-planned. It was a last-minute thing and she has dropped the show right in it. They are not happy," said the insider.