A scheme to encourage motorists to go green has been described as "useless" by an environmental group.

Haringey Council in north London is urging residents to switch to environmentally friendly fuel in the latest development to its pro-green tax incentive.

Those who convert their vehicles to liquefied petroleum gas (LPG) - a natural resource and an alternative to petrol and diesel - will pay less under the borough's new carbon emissions-based residential parking charges.

Cars run on LPG fuel give off significantly fewer carbon emissions than those using other fuels, and as a result the council has vowed to reduce parking permit charges by one banding grade if residents can prove their vehicles run on LPG fuel.

The carbon-based parking charges, rubber-stamped at the end of last month, include four band widths with charges ranging from £15 to £150 a year.

Unfortunately, it costs an average £1,600 to convert a car to LPG, and with the maximum saving on parking permits standing at £50 per year, it would take 32 years just to recoup the outlay.

This is also the best case scenario, and does not account for the fact that an LPG system requires an annual service which can only be carried out by a specialist approved installer, resulting in further costs.

Currently there is only one specialist refuelling station suitable for these cars in the borough.

Ofer Acoo, co-ordinator of campaign group Green N8, promoting an eco-friendly agenda in Haringey, does not believe enough people would take up this idea for it to make any significant difference to the borough's carbon footprint.

She said: "By the time you've got any financial reward, you will have got a new car and there will be new technologies. In every way you look at this, it's not going to make a difference.

"In order to create a shift in human behaviour, you have to give people a real incentive. Most people can't afford it and so they can't change their behaviour - it's useless. I don't think that any of the CPZ charging is motivated by a green agenda, it's a mere tax."

Councillor Martin Newton, Liberal Democrat spokesman for traffic issues, agrees there is no real incentive for residents.

He said: "While any initiative to reduce the damaging effect that cars have on the environment has to be welcomed, the facts also need to be taken into consideration. It costs on average £1,600 to convert a car to LPG, which means that it would take years for the saving on parking charges to cover this cost.

"In reality, how many residents will benefit from this initiative? What Haringey Council needs to provide is real incentives to encourage greener behaviour."

However, Councillor Brian Haley, executive member for environment and conservation, has affirmed the council's belief that the scheme will make an important difference.

He said: "The whole point of the new parking charges is to encourage residents to use vehicles with lower carbon emissions.

"One of the main points made during the consultation on the original proposals was that they didn't cater for people reducing emissions by converting to LPG. We fully accept that argument.

"We hope that our new discount scheme will encourage more residents to convert their vehicles to LPG and help us in our drive to tackle climate change at a local level."

Residents can apply from July 1 for parking permit savings if they can demonstrate their vehicle has been converted to LPG. For more information call the council on 020 8489 0000.