Transport for London has been asked to investigate a higher congestion charge for people in gas-guzzling cars.
Drivers must pay up by midnight, or face a £100 fine
Mayor Ken Livingstone has said he would like to see drivers producing twice the acceptable amount of CO2 paying twice the charge.
"There would be a huge price incentive to Londoners to buy and drive non or less-polluting cars," he said.
It follows the chancellor's moves to charge cars with the highest emissions more vehicle excise duty.
Mr Livingstone is no fan of London's legions of 4x4 drivers, branding them "complete idiots" in a television interview.
Some drivers point out that not all 4x4 vehicles produce high amounts of emissions.
Last week the Alliance Against Urban 4x4s handed a petition to the mayor, asking him to look at charging owners of cars producing more than 225g/km of carbon dioxide a day to pay a £20 charge.
Currently drivers pay £8 a day to drive into the central London congestion zone, which is due to be extended westwards in February 2007.