Davina McCall is swapping Big Brother for big blisters as she sets off on David Walliams's charity bike ride to pedal the length of Britain.

Pedal pusher: Davina McCall

The presenter will be joining a team of celebrities such as Fearne Cotton and Jimmy Carr who will tackle the mammoth John O'Groats to Land's End ride.

Gallery: Celeb cyclists gear up for BT Sport Relief Million Pound Bike Ride

The seven-strong group set off today and will endure a non-stop cycle relay to complete the 1,000-mile BT Sport Relief Million Pound Bike Ride.

They will have to battle the elements as they pedal hard day and night with the aim of completing the journey in around 80 hours.

Each of the competitors will take it in turns in the saddle while the others continue the journey on a tour bus, which will be their only relief for the four-day trip.

The team is completed by comics Patrick Kielty, Russell Howard and Miranda Hart.

Walliams, who previously swam the English Channel for Sport Relief in 2006, will be leading the attempt, and enlisted the others.

The Little Britain star, who last month became engaged to catwalk model Lara Stone, said: "It's a phenomenal undertaking and going to be really tough but we're all training hard to make sure we all get to Land's End and hopefully raise a million pounds for Sport Relief.

"I've seen what money raised through Sport Relief can do and it really does make a difference. It will be this keeping me going through the tough journey ahead."

Footage from their journey will form a documentary which will be broadcast by the BBC in the run-up to Sport Relief Weekend, from Friday March 19 to 21.

Details of sponsorship are at www.sportrelief.com and to help the fundraising push, BT will donate 1p for every call made from a BT and a Plusnet home landline on February 26, which it is declaring "Chat for Change" day.

Funds will help vulnerable people in the UK and some of the world's poorest countries.