Alarm about the potential environmental harm caused by mass-producing biofuels such as ethanol has reached a new height with the UK Government's own scientific adviser warning of the possible dangers.

Consequently, company executives should not rely on increased use of biofuels in vehicle fleets to reduce emissions, say environmental groups.

"The status of biofuels is debatable currently," said a spokesperson for Greenpeace. "We urge businesses to make vehicle fleets more green by buying more efficient vehicles, making more efficient use of vehicles - for example by rescheduling deliveries, and reducing journeys by using tele-conferencing."

Yesterday, Professor Robert Watson, chief scientific adviser at the Department of Environment, Food and Rural Affairs (Defra), joined the growing clamour of voices urging the government to delay the Renewable Transport Fuels Obligation (RTFO), a European Union ruling that demands fuel companies in all member states add 2.5 per cent biofuels to petrol and diesel as of 15 April this year.

The EU ruling further calls for petrol and diesel to comprise five per cent biofuel by 2010 with proposed extensions to 10 per cent by 2020.

The Renewable Fuels Agency, part of Defra, commissioned a full scientific review of biofuels on 21 March, but an initial report is not due to be presented to the Department for Transport and Brussels until 27 June, long after RTFO will have come into force.

A coalition of NGOs yesterday wrote to Transport Minister Ruth Kelly, urging her to delay the RFTO until the sustainability of biofuels can be proved or otherwise. Signatories included Friends of the Earth, Greenpeace, Oxfam, the International Institute for Environment and Development (IIED), Oneworld, The Catholic Agency for Overseas Development (CAFOD), Operation Noah and the Royal Society for the Protections of Birds (RSPB).

"Given the evidence that already exists, we believe that the introduction of biofuels targets will, in the absence of … a clear understanding of the indirect impacts of large-scale production of biofuels, have a devastating impact on vulnerable people's livelihoods, the climate and biodiversity," says the letter.

The environmental groups list various negative impacts of biofuel production, including: increased food prices as agricultural effort and land is diverted from food production to growing crops for biofuel; increased use of fossil-based fertilisers to grow biomass; release of carbon into the atmosphere by the ploughing of virgin land; and destruction of carbon-sink forest and wildlife habitat to make way for fuel farming.

Furthermore, a recent report in The Economist highlighted the huge fresh water consumption of ethanol plants, at a time when global fresh water reserves are under increasing pressure. According to the report a typical plant producing 50 million gallons of ethanol a year requires 500 gallons of water every minute.

In 2007 US President George Bush signed into legislation a requirement for a fivefold increase in biofuels production to 36 billion gallons by 2022.

However, environmentalists do not damn biofuels universally, pointing to the difference between the major drawbacks of producing ethanol from corn, which the US is pursuing, and the relatively benign Brazilian ethanol produced from sugarbeet plants.