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."
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The work, called Gift Horse, replaces the giant blue cockerel that previously sat on the plinth.
50 of the country's worst pollution blackspots are in the capital. Many in places you may visit every day.