London mayor Ken Livingstone has said Boris Johnson would be a "formidable" opponent, but warned against turning the contest into "Celebrity Big Mayor".
Mr Livingstone has been mayor since 2000
Mr Livingstone, who will stand for a third term next year, said the Tory MP was a "charming rogue" but he wanted to talk about potential policies.
Mr Johnson, one of the best known MPs, is the favourite of four Tories on the party shortlist for mayoral candidates.
Another high-profile MP, Lib Dem Lembit Opik, has ruled out entering the race.
Mr Livingstone told BBC Radio 4's Today programme he was worried about the media's approach to the mayoral contest, which was to look for characters with "good one liners".
"My job in the next eight months or so is to focus on what Boris actually believes in," he said.
He said he was "halfway through" reading all Mr Johnson's newspaper and magazine columns over the past 20 years - Mr Johnson was once editor of Spectator - and said his rival made right-wing Conservative peer Norman Tebbit "look like a cuddly liberal".
"I want to get onto the policy...I mean this is not a sort of Celebrity Big Mayor, it's a serious issue about how you run the city," he told the BBC.
He famously won his first term in 2000 as an independent after being expelled from the Labour Party but won his second term as the official Labour candidate after rejoining the party.
He was responsible for introducing the congestion charge in London but is equally well known as an outspoken anti-establishment figure.
Mr Johnson's decision to apply to become one of the Conservative candidates for the 2008 contest prompted a media scrum - the MP is a frequent guest on television shows and has a high media profile.
Mr Johnson's decision to run sparked a media scrum
Mr Livingstone told the BBC: "I know a lot of people say Boris will be a walkover. I think Boris is the most formidable opponent I will face in my political career.
"He's got a high profile, he's a charming and engaging rogue, that's why I want to get on to what he believes in and will do, rather than whether you would want him as your next door neighbour."
The Conservatives had long been searching for a big name candidate with a realistic chance of beating Mr Livingstone.
Previous possible runners have included Greg Dyke, Digby Jones, Nick Ferrari, Lord Stevens and Sir John Major.
On Wednesday there was speculation that Mr Opik, who hit the headlines in December 2006 with the news he was dating Cheeky Girl Gabriela Irimia, might run for the Liberal Democrats.
He later said he was "very flattered by the outpouring of affection" to convince him to run, but his first priority was to his Mid-Wales constituency.