Gary Barlow, Chris Evans, James May and Brian Cox have set off from Lands End in a pink Rolls Royce raising money for Breast Cancer Care.
The challenge will see them driving from Land's End to John O'Groats in a bespoke pink Rolls Royce Ghost.
They want to try and raise £1 million in 24 hours.
The four celebrities will stop off in Bristol as part of their trip.
More top news
Sunny spells, but cloudier this afternoon. Temperatures only reaching 7 or 8 degrees. Another widespread frost overnight.
Not as sunny as this morning, but still some bright spells. It's cold though, with temperatures around 8 or 9 degrees.
The company behind the Bristol Pound currency says it may have to stop offering its digital service if it can't secure further funding.