The Green Party says that London has been breaking European air pollution limits for nitrogen dioxide since 2010.
London Assembly Member Jenny Jones welcomed the announcement of an ultra-low emission zone - but urged the mayor to move much more quickly.
“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. We have argued for years that the Mayor’s list of little measures is inadequate and that an ultra low emission zone was essential.
"It's already too late for our breaking the European legal limits, but if we are to clean up London's dirty air we need this zone now, with a zero emission target. I urge the Mayor to be brave, drop his planned backward steps on the 2015 measures and make the big changes happen by the end of this year."
– Jenny Jones, Green Party member of the London Assembly.
"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.”
Boris Johnson says he intends to introduce the ultra-low emission zone from 2020.
By that time, City Hall says, all central London buses will be hybrid vehicles, zero-emission taxis will be running on the capital's streets, low-emission options for freight will be available and electric cars may be in far greater use.
The Mayor says his proposal will act as a challenge for car manufacturers to develop more affordable low-emission vehicles.