Fewer cars and faster speeds through London's congestion zone has reduced nitrogen oxides, particulates and carbon dioxides but London's mayor Ken Livingstone is not satisfied the benefits can be sustained and is proposing a £3 hike in the daily rate.

Mr Livingstone has asked Transport for London (TfL) to begin consulting on new charging system and rates for vehicles to enter the central zone.

The mayor is keen to maintain and build upon the significant traffic benefits that have arisen so far from the central London scheme. However, in time these benefits will be eroded if the value of the charge remains the same.

In return, the mayor intends to make the charge less "hassle-free" for motorists with discounts and easier payment schemes.

The congestion charge, which was introduced in February 2003, has been credited with reducing congestion in central London by 30% with 70,000 fewer vehicles entering the zone and cities across the world are keeping a close eye on the scheme. Edinburgh is due to hold a referendum next year, as the city will not meet National air quality targets by 2010 without stringent measures.

However successful the congestion charging scheme in London is acclaimed, such schemes are unlikely to reduce city pollution significantly without drastic additional measures. London's scheme covers only a mere 1.3% of the city and the small improvement in air quality gained was wiped out last year due to an unusually warm summer.