Mr Livingstone dubbed the Tory MP "a charming and engaging rogue" and predicted he would be "no walkover".

Mr Johnson - who is yet to be formally adopted by the Conservatives as their candidate - saw his bid to become Mayor boosted on Wednesday by an unofficial opinion poll suggesting he is in the lead among the capital's voters.

A small sample of London voters in a YouGov survey of voting intentions gave Mr Johnson a six-point lead over the Labour incumbent.

Speaking on BBC Radio 4's Today programme on Thursday, Mr Livingstone said: "My job in the next eight months or so is to get to focus on what Boris actually believes in.

"He has been writing three columns a week for 20 years. I am halfway through them."

And he added: "This man is to the right of Norman Tebbit - he makes Norman Tebbit look like a cuddly liberal.

"So I want to get on to the policy. This is not a sort of Celebrity Big Mayor. It's a serious issue about how you run the city.

"I am going to enjoy it, but 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 is a charming and engaging rogue.

"That's why I want to get on to what he actually believes in and what he will do rather than whether you would want him as your next door neighbour."