Ken Livingstone's multi-million pound plans for a "low emission zone" for greater London are to be scrutinised.
The mayor wants to reduce air pollution
The Mayor of London wants to discourage the most pollution-producing diesel vehicles - such as old lorries, buses and coaches - from entering the city.
It might mean vehicles which do not meet emission standards are charged to enter the LEZ, or banned from it.
The zone, aimed at improving health and helping meet UK and EU air quality targets, could be brought in by 2008.
Cars would not be included, nor would vehicles on the M25 London Orbital.
The mayor, who already fought off much opposition to his £8-a-day congestion charge scheme, has said he wants to make London the most environmentally-friendly city in the world.
But it is currently unlikely to meet its targets for reducing nitrogen dioxide and particulate matter - for which the mayor says road transport is responsible for about 47%.
The LEZ plans, which could cost up to £10m to set up depending on the scheme, will be scrutinised by the London Assembly on Tuesday.
Darren Johnson, chairman of the assembly's green committee, told BBC London: "We want to make sure that this scheme stacks up in terms of actually improving air quality and bringing London into line with EU laws.
"We also need to make sure it stacks up financially, that there is funding in place that businesses are able to switch over to cleaner vehicles... There's a lot of work to do."