Proposed changes to London's congestion charge that would see low emission vehicles exempted from the payment would have virtually no impact on pollution levels, according to a leading motor industry trade body.

The Society of Motor Manufacturers and Traders (SMMT) today called on Transport for London (TfL) to extend the consultation period for the new proposals, which ended on Friday, amid claims that the scheme would cut London's annual transport emissions by 8,100 tonnes, or just 0.084 per cent.

SMMT said its calculations were based on TfL's own figures and predicted that the scheme would lead to increased congestion as it would encourage more cars onto the capital's roads.

The SMMT added that car industry moves to cut average carbon emissions and reduce the carbon footprint of manufacturing processes would provide a more cost-effective means of cutting transport-related emissions.

"The motor industry has asked the mayor for an extension to the consultation period to work with Transport for London," said SMMT chief executive Christopher Macgowan. "The issues are complex and TfL must be absolutely clear about the scheme's aims. Its execution must also deliver the greatest benefit, both in terms of congestion and CO2 reduction and the charges to drivers must be proportionate."

However, a spokeswoman for TfL said there were no plans to re-open the consultation period and insisted that the company would carefully monitor the introduction of the scheme, should it go ahead, to ensure reduced congestion and CO2 benefits are maintained.

She added that the proposals were supported by 64 per cent of Londoners and argued that regulations such as the proposed congestion charge changes were required to encourage people to buy lower emission vehicles.

"Although emissions of new vehicle models are declining that rate of change is far too slow to tackle climate change," she said. "Meanwhile, sales of the most polluting vehicles are actually rising. The Emissions Related Congestion Charging proposals aim to deal with this directly, discouraging the use of the most polluting vehicles.

Under the proposed changes to the congestion charge, high emissions vehicles that emit more than 225g of CO2 per kilometre will be charged £25 to enter the congestion charge zone, while low emissions vehicles that emit less than 120g of CO2 per kilometre will be allowed to enter the zone for free.

Traditional car manufacturers find themselves in an unlikely alliance with new electric car companies – such as the G-Wiz and the Mega City Nice car – in criticising the proposed changes to the scheme. Manufacturers of electric cars – which can already enter the congestion charge for free – have opposed the changes on the grounds they will diminish the competitiveness of electric cars and encourage people to buy low emission vehicles that still release carbon instead of electric cars that have virtually zero emissions.

In related news, the European Parliament is tomorrow expected to vote on an amendment to proposed legally binding targets for average car emissions that would see targets watered down from 120g per kilometre by 2012 to 125g per kilometre by 2015.

The European Commission is expected to set out a formal legislative proposal at the end of the year with the new law expected to pass in 2009. Tomorrow's vote is being seen by environmentalists as a crucial test of parliament's commitment to impose stringent fuel efficiency standards.