London could have the world's first Ultra Low Emission Zone, according to Mayor Boris Johnson who has announced plans to improve air quality.
Only zero and low-emission vehicles would be allowed in the central London zone by 2020, said Mr Johnson.
The AA said it was an ambitious timetable and that the practical and economic impacts needed to be examined.
Clean Air In London said it was a wish list Mr Johnson wants others to do as he would only be mayor until 2016.
London is among the worst in Europe for air pollution.
Environmental Organisation ClientEarth has warned that in terms of European Union rules, London will have "illegal levels of air pollution until 2025".
A London Assembly report last December found that 9% of deaths in the city were down to air pollution.
In a speech to City Hall, Mr Johnson said: "Creating the world's first big city ultra low emission zone has the potential to be a game changing moment in the quality of life of our great capital.
"My vision is a central zone where almost all the vehicles running during working hours are either zero or low emission.
"This would deliver incredible benefits in air quality and stimulate the delivery and mass use of low emission technology."
City Hall said the mayor recognised that more work was required and believes his proposal would act as a challenge for vehicle manufacturers to ensure more affordable and low-emission technologies were in place.
Simon Birkett from Clean Air In London said the timescale was far too long and that it would only cover the central Congestion Zone.
"It's the biggest public health risk after smoking and we need bold action - he should have been starting on this in 2008 need bold action in two or three years, not seven or eight," he added.
Jenny Jones, the Green Party's London Assembly member, said the scheme was first put to the mayor by the Assembly in 2009.
She added that London had been in breach of European guidelines for pollutant nitrogen dioxide since 2010.
"This is excellent news for Londoners' health, but leaving it as a project for the next mayor to deliver is a way of ducking responsibility for the problems we're facing now," she said.
A spokesman for the AA said it was a tight timetable and questioned what impact this would have on police cars, ambulances, fire engines and vehicles used for essential deliveries.
He added that decisions made now would be affected by this change such as companies buying a fleet of taxis would need to know if the vehicles would not be allowed in the zone in seven years.
The mayor also announced that phase five of the Low Emission Zone would now only apply to TfL buses, which would save £350m in costs for businesses.
Mr Birkett said this was a backward step, especially as the low emission zone was the mayor's own policy.
The mayor also plans to introduce 1,600 hybrid buses by 2016 and a further £20m fund to help boroughs tackle local air quality hotspots.