Controversial regulations that require all petrol and diesel sold on UK forecourts to contain a small amount of "green" fuel are coming into force.
The Renewable Transport Fuels Obligation (RTFO) is being introduced with the aim of reducing the climate change emissions from transport - which produces more than a quarter of overall greenhouse gases in the UK.
Suppliers are required to source a small, but increasing, percentage of fuel from crops such as palm oil, maize and sugar cane.
The crops absorb carbon dioxide as they grow and should in principle have lower overall emissions than petrol or diesel.
But the policy, which initially requires a 2.5% renewables content in all transport fuel, comes into force amid accusations biofuels are contributing to climate change, human rights abuses and environmental destruction.
Aid agency Oxfam said the UN Permanent Forum on Indigenous Issues had estimated some 60 million indigenous people faced clearance from their land to make way for biofuel plantations such as palm oil.
There are also concerns the demand for biofuels is driving food prices up and leading to human rights abuses and slave labour on plantations, Oxfam said.
The RSPB said forest clearance, use of fertilisers which produce greenhouse gas nitrous oxide and the energy used to convert crops to fuel and then transport them could all make the overall emissions of biofuels higher than their oil or diesel equivalents.
But farming leaders said different biofuel production systems had hugely different impacts on the environment, greenhouse gas emissions and whether they compete for land with food.
Sustainable biofuels grown in Britain from feed wheat that would otherwise be exported, or oilseed rape on set-aside land, could be produced in large enough quantities to meet the RTFO, National Farmers' Union president Peter Kendall said.