London Assembly Labour environmental spokesman Murad Qureshi said Johnson's decision was "fatally bad news" for public health.
"It is shameful, but no big surprise, that Boris Johnson has attempted to bury his bad news under [yesterday's] snow," said Qureshi.
"London's air quality is already the worst in the UK and is responsible for over 1,000 deaths a year - more than are killed on our roads. Londoners will simply not believe that the Mayor is serious about improving the environment they live in while he continues to take such regressive decisions; decisions that are fatally bad for their health.
"I hope that the Mayor will look again at this and consider options like a mini low emissions zones on the London's most polluted roads. These are just the kind of issues we will be looking at on Wednesday when the environment committee meets."
Phases one and two of the Low Emission Zone currently target the most polluting lorries over 3.5 tonnes, buses and coaches. Phase three of the scheme, planned by the previous Mayoral administration, was scheduled to start in October 2010 and would have affected much smaller vehicles, including vans and minibuses, and even family camper vans.
"I want to do all I can to ease the burden of the economic downturn that is affecting us all at this time," said Johnson.
"Although the Low Emission Zone has been successful in tackling the worst polluters, and will continue to play an important role, it is not the right time to press ahead with extending it to include smaller vehicles like vans and minibuses.
"Many of these will be owned by small businesses, charities, and self-employed Londoners already hard hit by the recession. Simply put, the cost of fitting pollution equipment or getting a new vehicle would have come as punch in the ribs to those who need our help at this time, would have destroyed profit margins, and endangered our businesses. I am confident that the new course we have set finds a balance between London's environmental and economic needs and that we can lower emissions in more imaginative ways."