Under the proposed exemptions, only vehicles that run solely on liquefied petroleum gas will be exempt. Petrol vehicles converted to run on LPG will still have to pay £5 a day to enter the charging zone.
But the Government-funded TransportAction PowerShift programme said the current proposals to exempt only mono-fuelled vehicles would mean virtually no vehicles would qualify for the exemption.
Jonathan Murray, director of TransportAction, said the list of vehicles exempted from the scheme had to be widened to include bi-fuel and hybrids.
His comments were underlined by Darren Johnson, Livingstone's former environment advisor and current leader of the Green Group on the Greater London Authority.
Johnson said bi-fuel vehicles should be included in the list of vehicles exempt from the charge and called on Livingstone to acknowledge the realities of the marketplace and allow concessions to bi-fuel vehicles.
He warned: 'Excluding bi-fuel vehicles from the list of exemptions will undermine the work done so far to introduce LPG and will mitigate against the development of production line LPG vehicles.'
A Transport for London spokesman stressed that the list of vehicles to be exempt from the charges was still at the proposal stage. But he added the scheme was aimed at reducing vehicle congestion in London and not at improving air quality.