Boris Johnson's refusal to endorse cartoons shows the mayor has had a "sense of humour failure", the campaign group behind the posters said.
Clean Air London has released a series of cartoons featuring the mayor as he tries to deal with some of "most important air pollution themes" in the capital.
“We asked the Mayor to endorse the cartoons to stimulate debate on many of the most important air pollution themes in London," Simon Birkett, Founder and Director of Clean Air in London, said.
"Amazingly, Boris’s refusal to do so has done more to promote the cartoons than his endorsement would have done. People may now be laughing as much at Boris’s sense of humour failure as the wonderful cartoons themselves."
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