Six riders will start the fund-raising effort, with comic Jimmy Carr joining later
Six celebrities have begun cycling the length of Britain in a bid to raise £1m for Sport Relief.
Radio 1 DJ Fearne Cotton, comics David Walliams, Miranda Hart, Russell Howard and Patrick Kielty and TV presenter Davina McCall are taking part.
The stars are participating in a non-stop relay from John O'Groats on the tip of Scotland to Land's End, the most westerly point in England.
They will be joined en route by a seventh rider - comedian Jimmy Carr.
Walliams on the challenge ahead
The first 24 hours of the ride will see the celebrities take on an overall hill ascent of almost 29,000ft - the equivalent of Mount Everest.
Cotton said: "When I was told that we would be climbing the equivalent of Mount Everest on the first day alone, I thought it was a joke.
"But when you look at the route, it's true - there's almost 29,000ft for us to pedal up on the Scottish leg of the trip. And a lot of that is in the dark!"
Little Britain star Walliams, who swam the English Channel for Sport Relief four years ago, said: "Water is one thing, but I'm not built for a bike. I'm not sure what I've let myself in for."
Sport Relief, a spin-off of the long-standing charity Comic Relief, takes place from 19 to 21 March and will raise money for charity schemes in the UK as well as developing countries.
It will kick off with a night of entertainment on the BBC, with a sketch from Gavin and Stacey star James Corden and special editions of Masterchef, Top Gear and A Question Of Sport.
Other celebrity challenges include rugby ace Lawrence Dallaglio cycling around the Six Nations and BBC One Show presenter Christine Bleakley water skiing across the English Channel.