City Hall has declined to comment on the release of a series of cartoons mocking Boris Johnson's handling of London's environmental problems.
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."
A series of posters mocking Boris Johnson for his response to the capital's pollution problems have been released by an environmental campaign group.
The images, commissioned by Clean Air London, all feature the Mayor of London as he attempts to address the key environmental issues affecting Londoners.
Poor air quality is said to contribute to more than 4,000 premature deaths in the capital every year.
Boris Johnson was accused of "dangerous complacency" after he claimed London's air quality seemed "fine" during "very high" levels of pollution earlier this month.