YouTube star Zoella will join A-listers like Michael Sheen, Dame Edna Everage and Joanna Lumley in a Comic Relief edition of The Great British Bake Off.
The 24-year-old, whose real name is Zoe Sugg, will drag down the average age of the competitors, who also include Jonathan Ross, Lulu, and former Prime Minister Gordon Brown's wife Sarah.
"I was genuinely a little bit shocked and very excited when I was asked to take part," she said.
The series begins on 11 February.
Taking place over four weeks, it will see 16 celebrities compete to be named star baker by show stalwarts Mary Berry and Paul Hollywood.
The line-up features former Radio 1 DJ Chris Moyles, fashionista Alexa Chung, comedian David Mitchell and TV presenter Gok Wan.
They are joined by DJ Jameela Jamil, comediennes Victoria Wood and Jennifer Saunders, Strictly champion Abbey Clancy and Kayvan Novak, from Channel 4's Fonejacker series.
Mary Berry said it had been "great fun" filming the series.
"Although there were some baking catastrophes, some surprising hidden skills were revealed," she added.
"One thing for certain is that anyone can have a go at baking and it's a great way to raise money for Comic Relief - and you are guaranteed to have a lot of fun!"
Regular presenters Sue Perkins and Mel Giedroyc will be joined by Jo Brand and Ed Byrne for the series, while an episode of spin-off show An Extra Slice will be broadcast on Red Nose Day itself, Friday, 13 March.
The bakery show has proved a great success, spawning a US spin-off and recently winning a National Television Award for "best skills-based reality series".
But Perkins recently revealed she had twice turned down the opportunity to work on the show.
"I have no ability to work out what is a good thing to do or not," she told Jonathan Ross on his ITV chat show. "I said no because [it] sounded ludicrous".
She added that the series had often turned contestants into divas.
"Sometimes television changes people," she said.
"It's amazing how quickly they turn," added her co-host, Giedroyc.
"Week one, they're all lovely. Week three, 'oh darling, this bowl isn't big enough.'"