The zone was first announced in February 2013 and will require vehicles entering central London to emit zero or low emissions.
Drivers of vehicles which don’t comply will receive a daily fine.
Assembly Members want the Mayor to introduce the scheme sooner to help reduce the 4,000 premature deaths each year attributed to poor air quality.
Responding to the Mayor’s consultation on the ULEZ, the Assembly’s Environment Committee also wants Mr Johnson to work with local boroughs to widen the scheme and ensure emissions standards are regularly reviewed as cleaner vehicles become available.
The Mayor has previously defended the 2020 start date saying it would be wrong to force vehicle owners to upgrade until there are more low and zero emission vehicles on the market.
Committee chair, Stephen Knight AM, said: “The Mayor’s argument that owners need time to adjust their purchasing decisions doesn’t wash because non-compliant vehicles will not become unusable, they will just be subject to charges.
“TfL has said that nearly three quarters of the traffic in central London will meet the proposed ULEZ standards by 2020 even without the zone.
“The financial costs to a small number of drivers must therefore be weighed against the worrying number of Londoners affected by respiratory problems and thousands of early deaths linked to the capital’s air quality.”
