A report by campaign groups has warned that burning biomass (such as wood) in power stations may hinder attempts to tackle climate change.
Biomass is expected to contribute about third of the UK's mandatory EU target for renewable energy by 2020.
It's described by the Committee on Climate Change as an economic means of low-carbon power.
But the report warns it will take too long for trees to re-absorb the carbon emitted by burning wood.
It also expresses concerns over the scale of the plans.
The government has opened a consultation asking how much carbon can be saved by burning biomass (plant material) and whether the policy will harm forests.
Biomass burning is not a zero-pollution option. It creates greenhouse gases to cut and transport the wood, and when the wood is burned. But supporters say that so long as the burned vegetation is replaced by new plants to absorb CO2 that should confer a significant advantage over using fossil fuels.
As biomass burning expands the topic is increasingly controversial. Drax power station - the UK's biggest source of electricity - is converting three of its six giant boilers to burning biomass. They will gobble up nearly seven million tonnes of plant material a year.
Drax will have to import 90% of its biomass. The firm says its major source will be unwanted offcuts from the timber industry, mainly in the Americas.
From 2013, the government mandates that biomass burning for power will need to emit no more than 70g CO2/kJ after a lifecycle analysis including emissions from transport and cutting.
Drax says it averages between 20 and 75g, depending on the biomass used. The figures compare with 280g for the average UK coal power station (Environment Agency); 122 for North Sea gas; and 193g for Russian piped gas (Friends of the Earth).
But campaign groups are highly sceptical. They say the methodology is flawed.
Harry Huyton from the Royal Society for the Protection of Birds (RSPB) told BBC News: "Drax's demand for biomass will be huge - more than the entire output of forests in the UK. The power firms say they are using offcuts, but there are some whole trees going into the system and as demand expands we simply don't believe that forests and wildlife will be protected.
"We have seen the policy on biofuels for road transport go horribly wrong. We don't think biomass burning is as foolish as biofuels policy - but we have major misgivings about biomass policy too. Do we really want to be shipping wood to burn from America?
"Of course growing plants absorb CO2 - but when they're burned it releases CO2 immediately and you have to wait for decades or hundreds of years for that to be taken up. With climate change we don't have the time to waste."
The power firms say it's important to look at the detail of their proposals. Drax claims that because shipping is so efficient, it creates less CO2 bringing a boat-load of wood from America - even the West Coast - than it does ferrying the equivalent wood by lorry from forests in Scotland.
The wood from the West Coast of America is diseased and useless for timber, they say. It could, of course, be burned in the US to make power - but shale gas is so cheap that it's not worthwhile.
If the offcuts weren't fed into power stations, they would be burned as waste or left on the ground to decay, producing methane and CO2.
So long as the capacity of the forests to absorb CO2 outweighs the amount released, there will still be a carbon credit. The system can be monitored by satellite and by measuring trees in the forest, Drax says.
Nigel Burdett, head of environment at Drax, told us: "The NGOs are looking at worst practice, burning whole trees.
"But timber is far too high a value for us to burn. We take sawmill residue that used to be burned to get rid of it. Much of the wood we use has been destroyed by pests, so it needs to be disposed of anyway.
"It's not zero carbon but we are confident that most of the material we use (for burning) are within our target of 80% carbon savings."
Dorothy Thompson, chief executive of Drax, told BBC News that biomass was the ideal fuel to balance variable wind power. "It is totally complementary - we can be flexible and reliable. Wind is a good energy source but it's intermittent - we can fill that gap."
Campaign groups agree that it is better to use offcuts as a resource rather than let them rot, but the potential scale of the enterprise alarms them as they believe in future power generators will be scouring the globe for stuff to put in the furnace.
The government is still working through its long-term policy on the issue. The latest subsidy changes mean that for many firms it's not worthwhile building bespoke biomass power stations, but it is worthwhile converting some existing coal-burning stock.
The government expects subsidy for biomass to be between £442m and £736m in 2016/17. Extra biomass generation after that will be supported by a new subsidy mechanism known as CfDs (Contracts for Difference). These are still being discussed but an industry source said that Drax alone would expect at least to recoup its £750,000 investment in converting boilers to burn wood.
The Department of Energy and Climate Change's (DECC) Renewables Roadmap estimates that biomass could potentially provide between 68 teraWatt hours (TWh) and 100TWh of renewable heat and electricity by 2020, which is 26-42% of the total needed to meet the EU renewables target.
A DECC spokesman told BBC News: "We'd stress that investment in biomass brings diversity to the (energy) mix, it's flexible so can be used as back up to intermittent sources of generation, and it's clean. We're also making sure it's sustainable."
David Kennedy, chief executive of the Committee on Climate Change said: "Biomass burning is a sensitive thing because it's an economic low-carbon source of energy - especially relative to offshore wind."
What's uncertain is the role for British farmers in this great enterprise. A few years ago they were looking for profits in energy crops like miscanthus - elephant grass - and coppiced willow.
Drax will get 10% of its fuel from British sources like these and there's a high-walled maze at the power station of blocks of chopped miscanthus next to high mounds of shredded willow.
But since energy crops first stirred agricultural excitement there's been controversy over the impact of using land to grow crops to burn. Drax say miscanthus can still be useful for farmers wanting to stabilise sandy soils.
But it is possible that in future this may be uneconomic, and that growing coppiced wood for burning will be mainly concentrated on the high-value market for wood-burning stoves in the homes of the rich.
In the meantime, the imports of wood from round the world are likely to continue.