Bumbling Boris Johnson has been branded a "clown" by his Lib Dem rival.
Brian Paddick, who is the Lib Dem's candidate for London Mayor said the Tory would be embarrass the capital and offend Londoners.
Balham-born Brian Paddick who was also the former Met Police commander, made the comments in an interview for website ePolitix.com.
In the interview the 49-year-old said: "Boris is a very bright guy - you don't get into Eton and Oxford by being stupid. But Boris has got into the habit of believing that nobody will like him unless he makes a joke.
"There are these accusations of him being racist and him being sexist, but at the end of the day he just can't help cracking jokes and quite often they're offensive to the people that he's making the joke about. And we need a serious ambassador for London to be our mayor - not a clown."
But Boris hit back in the an interview with The Times saying: "Londoners don't want to hear this kind of politically correct psycho-twaddle. I am offering a positive campaign to oust Livingstone, restore security to our streets, buses and station platforms and deliver taxpayer value."
Mr Paddick, who before he retired as deputy assistant commissioner in April had been the Met's highest ranking openly gay copper, beat challenges from Chamali Fernando and Fiyaz Mughal to take the Lib Dem nomination.
He said his main aims were less crime, better transport, cleaner air and fewer Londoners living in poverty.
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