Brand's ambition comes despite previously saying: 'Don't bother voting. We know it's not going to make any difference.'
The comedian - who is dating make up artist and mother-of-one Nicola Schuller after breaking up from heiress Jemima Khan - has never hidden his loathing of the Conservatives. Last year he wrote that Margaret Thatcher's death was only sad for 'a handful of people', and in his book, he calls David Cameron the most offensive four-letter word.
Brand's popularity among the young is reflected by his 8.4 million Twitter followers. But his outrageous comments and antics, such as leaving obscene telephone messages for Fawlty Towers actor Andrew Sachs in a radio stunt with Jonathan Ross, have caused deep outrage.
With nominations for the next London mayoral election not likely to close until five weeks before the contest in May 2016, Mr Brand has plenty of time to decide.
But any serious campaign would need to begin sooner, not least to raise funds. Mr Johnson raised £1 million for his 2008 campaign.
The BBC came under fire last week for giving Brand a platform on Newsnight for his extreme views, while the comedian said the BBC had an 'anti-Islamic narrative' and accused host Evan Davis of being 'mates with CEOs and big business' and 'cosying up' to Boris Johnson.
He previously appeared on Newsnight last year, but now admits he was 'policy bare' when interviewed by Jeremy Paxman.
He is not the only comedian hoping to become London Mayor, as Eddie Izzard has declared his ambition to stand as Labour's candidate in 2020.
- Mail on Sunday