Ken Livingstone has accused the Prime Minister of spending too much time "promoting himself".
The former Labour Mayor of London said "public schoolboy" Boris Johnson should be spending more time in Downing Street "sorting out the mess".
"If you become Prime Minister you need to be in Downing Street sorting out the mess and creating a proper economic strategy to get you forward," he told talkRADIO's Julia Hartley-Brewer.
Mr Livingstone, who was replaced as London Mayor by Mr Johnson in 2008, accused his successor of doing "virtually nothing" for the capital during his eight-year tenure.
"He spent all his time promoting himself, and if you look, the three months he has been Prime Minister, every day he is there on the television doing some PR stunt," he added.
"In the first two years I was Mayor, journalists were complaining 'Where is he?'. But I was actually in the office doing the job."
His comments come as Mr Johnson - who has been Prime Minister for 100 days - prepares for a general election campaign, ahead of a vote on December 12.
His rival, Jeremy Corbyn, is already getting things underway, and is today expected to slate the "born-to-rule Conservatives" in a speech in London.
Mr Johnson has blamed the opposition leader for his failure to deliver on his “do or die” promise to leave the European Union on October 31.
He said Mr Corbyn “refused to allow” Britain to leave and was causing more uncertainty for families and business.
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