Biofuels were once seen as the perfect way to make transport carbon-free, but a series of EU studies are throwing increasing doubt on the green credentials of the alternative fuel.

The latest to be released gave a preliminary assessment that biodiesel from soybeans could create four times more climate-warming emissions than conventional diesel.

The European Commission has not helped itself by keeping many of the studies hidden -- the most recent being an annex cut from a published report that was only released after Reuters and several NGOs used transparency laws to gain access.

Two other studies and leaked emails have added to the dossier of worrying evidence.

At the heart of the debate is an issue drily referred to as "indirect land use change". In short, that means that biofuels use land and soak up grain supplies, sending reverberations through world commodity markets.