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.
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Bristol Mayor George Ferguson has been accused of a "nasty personal attack" in a row over how the city's Green Capital cash was spent.
Bristol Mayor George Ferguson says there will be "further breakdown" of how the city's Green Capital cash was spent.