Plans to introduce a £25-a-day charge for the most polluting vehicles entering London's congestion zone look set to be scrapped after Boris Johnson was last week elected mayor.

The newly elected mayor, who begins his first day at work today, said in his manifesto that he would abandon Ken Livingstone's plans for a £25-a-day charge on 4x4s and other high-emission vehicles emitting over 225g per km of CO2. Signalling that he was set to make good on that pledge, Johnson was quoted at the weekend as saying he would stop Londoners being "clobbered" by the congestion charge, an initiative he described as "the most vicious fines of any civilisation yet known".

Plans for a westward extension of the zone are also under threat, after Johnson said in his manifesto that he would put up the proposals for a second public consultation and scrap them if residents were opposed. He has also said he wants to make it easier for drivers to pay the existing £8-a-day charge by allowing them to pay it on a monthly basis.

The move is likely to be roundly condemned by environmentalists and will raise concerns that Johnson will look to water down the congestion charge further.

The congestion charge has been London's flagship environmental policy since its launch five years ago and has since been emulated by a number of cities around the world such as Singapore and Stockholm.

According to figures from Transport for London, the scheme has cut traffic within the zone by 21 per cent since it was introduced and reduced congestion by eight per cent. Meanwhile, there has been a 45 per cent increase in bus use and a 43 per cent rise in cycling within the zone.

Advocates of the proposed £25-a-day charge, which was due to be introduced in October, claimed that with 17 per cent of cars entering the zone each day emitting over 225g of CO2 per km, it would have delivered a siginficant reduction in carbon emissions. They also argued that it would have provided an incentive for people to buy lower-emission vehicles, which under the proposed changes would have been exempted from the charge.

However, critics of the proposals – including car manufacturer Porsche which had begun legal action against the scheme – argued that it would have minimal impact on pollution and could even lead to an increase in emissions by encourgaing owners of the most polluting vehicles to buy smaller cars in addition to their larger vehicles.

Environmentalists and green business leaders will now be watching Johnson's first 100 days in office closely to see how he handles the capital's other climate change initiatives, such as Livingstone's recently announced green building programme and his plans to expand cycling lanes.

Despite his committment to abandon the new congestion charge, Johnson has been a strong supporter of Tory leader David Cameron's attempts to reposition the party as environmentally responsible and is also expected to introduce a number of new green policies in the coming months.

In an eye-catching manifest commitment Johnson said he would scrap Ken Livingstone's free newspaper, The Londoner, and invest the £1m a year saved in planting 10,000 street trees across the capital. He has also proposed an incentive scheme to reward householders who recycle with vouchers that they will be able to exchange for goods and services.