The UK's largest bioenergy supplier has been told by its own scientific advisers to stop calling biomass 'carbon neutral' - raising "difficult questions" about the future of the controversial energy form when its subsidies expire in 2027.

Drax began burning woody biomass pellets instead of coal to produce electricity ten years ago, on the basis that doing so effectively neutralises the planet-heating carbon emissions, because new trees are planted to absorb those gases.

This science is disputed, but the UK government classes bioenergy as renewable. This qualifies Drax for subsidies of around £1.7 million a day for providing about 6% of the country's electricity.

Drax has now been told by its independent advisory board to "reassess its criteria for determining carbon neutrality", according to a summary of meetings and correspondence last year.

"Drax should move away from saying 'carbon stocks are increasing/stable' and stating biomass is carbon neutral," added the board, chaired by former government chief scientific adviser Professor Sir John Beddington.

It comes as Drax meets for its AGM today.

It will be hoping for a hint of further subsidies in the government's long overdue strategy on biomass - fuels made from trees and crops, usually from overseas - expected by the end of June.

"Deeply worrying" but "nuanced"

Energy policy professor Rob Gross, who directs the UK Energy Research Centre, called it "surprising" if Drax has not yet "nailed down the criteria".

"The premise on which Drax was converted to bioenergy, and the basis on which it's been given government subsidies was that bioenergy is zero carbon, or at least very low carbon, and can contribute towards our carbon targets and net zero", he said.

A spokesperson for Drax said: "The science that underpins our approach is complicated, nuanced and evolves, and we take our responsibility to continue to develop our explanation of it very seriously."

The advisory board "reaffirmed the importance of our ongoing work to expand our use of science and evidence in how we discuss our carbon accounting and biomass more generally", they added.

Last summer Drax denied complaints to a UK government body that its claims to carbon neutrality were misleading and breached OECD guidelines.

Phil MacDonald, from energy thinktank Ember, described the findings as "deeply worrying".

He said Drax has "had more than a decade of operations to get its argument straight on why burning biomass is good for the climate".

If biomass were not carbon neutral, some emissions may have to be added to the UK's tally.

This prospect would likely be unattractive to the UK government, which is off course to meet its goals to slash emissions and eventually reach net zero by 2050.

'Difficult questions about biomass future'

The board's recommendation "raises difficult questions about the future of biomass beyond 2027" when subsidies expire, said Dr Dan Quiggin, senior research fellow at thinktank Chatham House.

Ministers are under pressure not to renew subsidies for bioenergy generators amid some concern that burning wood to generate electricity damages forests, soils and habitats, and may even increase carbon dioxide emissions.

The United Nations' climate scientists, the IPCC, said bioenergy can lead to an increase or decrease in emissions, depending on how sustainably the material is sourced, transported and so on.

Drax and the government say its operations follow the strictest sustainability criteria, and assesses its carbon impact using agreed IPCC methods.

"We can live without Drax because we can live without any individual power station," said Professor Gross. But whether we would want to is another question, he said, especially as the UK reassesses its energy security.

"It's not reliant upon the gas supplies from other countries, and, unlike wind and solar, it's flexible. You can turn it up, you can turn it down, you can turn it off."

Drax has warned its operations may become unviable after 2027, unless the government pledges further subsidies to help it develop bioenergy with carbon capture and storage (BECCS), which involves storing the emissions from the plant underground, in order to offer negative emissions.

'Preventing runaway climate change harder to achieve'

The "concerning" part, according to Dr Quiggin, is that if bioenergy is not carbon neutral to start with, it reduces the impact of adding on carbon capture and storage (CCS), because some of the carbon removals would be spent on offsetting those emissions.

"The carbon negative emissions of BECCS in the future could be lower, and that will mean that meeting our climate targets and preventing runaway climate change is that much harder to achieve," he said.

The government's climate advisers, the CCC, says the UK will rely on negative emissions, or carbon removals to meet climate targets, because some sectors like aviation might be impossible to rid of emissions.

A spokesperson for the Department for Energy Security and Net Zero said: "The Government, with support from the Committee for Climate Change, considers biomass a critical renewable and low carbon energy source.

"It has helped to dramatically reduce the use of fossil fuels and our exposure to volatile global gas prices, and provided an additional source of energy to bolster our energy security."

A Drax spokesperson added the company is "committed to ensuring that the biomass we source delivers positive outcomes for the climate, nature and the communities in which we operate".