New rules requiring the inclusion of biofuels in vehicle fuels come into force this week amid calls from environmentalists to find more sustainable ways to cut emissions from transport.
The introduction of the Renewable Transport Fuels Obligation (RTFO) on Tuesday will mean that all petrol sold in the UK will have to include at least 2.5% biofuels, rising to 5% by 2010.
But some scientists and green groups have voiced fears that far from being the silver bullet to solve climate change, they actually contribute more greenhouse gases through deforestation and the use of fertilisers than they save.
There are also concerns the switch to energy crops from food production - including a large-scale drive in the US to produce bioethanol from maize - is contributing to rising fuel prices.
Ahead of the RTFO's introduction, the Government insisted the gradual introduction of biofuels will cut millions of tonnes of carbon dioxide, and said it would not go beyond the 5% target unless it was sure it could be done sustainably.
But Friends of the Earth demanded transport's greenhouse gases, which account for around 28% of overall UK emissions, be tackled by investing in better public transport and mandatory emissions limits on cars.
A survey for the environmental group showed almost nine out of 10 people did not know that the renewable fuels - made from crops such as sugar cane or maize - would be required in their vehicles.
Of the 55% of those questioned who knew what biofuels were, fewer than one in seven thought they were the best way to reduce emissions from road transport, the YouGov poll found.
More than three quarters (78%) of the more than 2,000 adults polled agreed that European leaders should insist manufacturers double the fuel efficiency on new cars by 2020 and more than two thirds (70%) thought that the UK Government is not doing enough to improve public transport.
Friends of the Earth also said two thirds were not aware that biofuels could be contributing to the destruction of rainforests cleared to make way for growing the crops.