London mayor Ken Livingstone has said today that he hopes to approve plans for a £25 congestion charge on the most polluting vehicles as he seeks to position the environment as a major factor in the mayoral election.

Writing in the New Statesman, Livingstone said he would take a decision on the £25-a-day charge in the next month following the close of the consultation period. He predicted that it would become a key issue at the upcoming mayoral election, adding that on the environment the choice between himself and his main rival Boris Johnson was particularly stark.

"Johnson is one of the few politicians who supported George W Bush in opposing the Kyoto treaty," he added.

If, as expected, the proposals are approved the scheme will come into effect by 2009. SUVs and other high emission vehicles emitting over 225g per km of CO2 will be charged £25 to enter the congestion zone, while vehicles in road tax bands A and B that emit less than 120g per km will; become exempt from the charge. Cars in bands C, D, E and F will continue to be charged £8 a day.

The move is expected to heap further pressure on commuters and firms to shun high emission vehicles and instead opt for more fuel efficient vehicles.

The proposals have been broadly welcomed by environmental groups, although supporters of electric cars, which are currently exempt from the congestion charge, have argued that by extending congestion charge exemption to include low emission vehicles the mayor will undermine one of the big selling points for electric cars.