Biomass power - such as burning wood for energy - could do more harm than good in the battle to reduce greenhouse gases, the Britain's Environment Agency warns.
Ploughing up pasture to plant energy crops could produce more CO2 by 2030 than burning fossil fuels, if not done in a sustainable way, it said.
Its study found waste wood and MDF produced the lowest emissions, unlike willow, poplar and oil seed rape.
The EA wants biomass companies to report all greenhouse gas emissions.
The agency is calling on the government to introduce mandatory reporting of greenhouse gas emissions from publicly-subsidised biomass facilities, to help work out if minimum standards need to be introduced.
Wood-burning stoves, boilers and even power stations are seen by many as critical to Britain's renewable energy targets.
Biomass is considered low carbon as long as what is burnt is replaced by new growth, and harvesting and transport do not use too much fuel.