Senior executives at Drax raised concerns internally about the validity of the energy company’s sustainability claims while it publicly denied allegations that it was cutting down environmentally important forests for fuel, court documents have revealed.

Britain’s biggest power plant assured ministers and civil servants of the company’s green credentials as it scrambled to defend itself against claims in a BBC Panorama documentary that it had burned wood sourced from “old-growth” forests in Canada.

The company’s senior leaders, including its chief executive, publicly denied the allegations, but other executives at the North Yorkshire plant privately raised concerns that it did not have sufficient evidence to back up the sustainability claims, according to evidence submitted to an employment tribunal involving its former top lobbyist.

The owners of Drax have received more than £7bn in subsidies levied on household energy bills on the condition that the biomass pellets are made from waste or low-value wood from sustainable forests.

However, the company has faced repeated scepticism over the sustainability of its business model, which involves importing millions of tonnes of wood pellets across the Atlantic every year.

Drax denied the findings in the BBC’s documentary, broadcast in October 2022. The programme focused on a pair of pellet production sites in British Columbia, and the company accused the broadcaster of repeating “inaccurate claims about biomass” from “ill-informed” critics.

In the days following the broadcast, the company’s chief executive, Will Gardiner, responded to a letter from Jacob Rees-Mogg, then energy secretary, which raised questions about the documentary’s findings. He assured the government that Drax was complying with its subsidy requirements. A senior policy manager offered similar assurances to civil servants.

Gardiner wrote: “I have positioned sustainably sourced biomass to be at the heart of Drax. This requires careful and robust governance and traceability.”

However, court documents provided to several news organisations, including the Guardian, have raised fresh questions about whether the guarantees given by Drax to ministers, civil servants and the industry regulator Ofgem after the broadcast could be justified at the time.

Drax responded to the BBC documentary by claiming that 80% of the material used to make its biomass pellets was sawmill residue – “sawdust, wood chips and bark left over when the timber is processed” – and that the rest was waste material.

The Guardian revealed last year that forestry experts believe the company has continued as recently as last summer to source 250-year-old trees from some of Canada’s oldest forests via the Burns Lake pellet plant.

Rowaa Ahmar, the company’s former head of public affairs, took Drax to court alleging that she was sacked after writing to Gardiner in the weeks following the broadcast to warn that the company was “misleading the public, government and its regulator” about the sustainability of the imported pellets.

Ahmar gave evidence to the tribunal that the BBC’s allegations against Drax triggered a “level of chaos that I have never seen before”, and that her work after the documentary’s revelations demonstrated that the allegations “were correct and that Drax had been misleading the public, government and its regulator”.

Her witness statement claims that Drax’s head of compliance admitted in an email to colleagues in the days after the broadcast that the company might have burnt old-growth pellets from Meadowbank and Burns Lake “consistently” at its North Yorkshire plant since “at least 2019”.

The compliance boss warned that, if this were true, Drax would have committed a “significant misreporting of burn data” under the government’s subsidy schemes, according to Ahmar’s witness statement.

Ahmar’s statement claims that, in the weeks that followed, the compliance chief explained during an in-person meeting with Ahmar that Drax did not have enough data to prove the exact origin of all its wood pellets, meaning that the company could not prove that its biomass was sustainable and legal under the government’s requirements.

Paul Sheffield, the company’s chief commercial officer, said he had also been aware of these concerns. He said in a witness statement that he had been aware that the head of compliance “had some concerns about our ability to fully evidence the points we were making around Panorama” and that these concerns were later raised with Drax’s executive committee.

Further concerns were raised in an online meeting between Ahmar and a second member of the company’s compliance team, according to Ahmar’s statement, in which the staff member explained that Drax did not measure every log used for biomass sustainability but that “we represent to Ofgem that we do”. The tribunal documents do not suggest that Gardiner was aware of the concerns raised by some Drax executives when he first denied the BBC’s allegations.

Gardiner said in his witness statement that he had been advised by his head of corporate affairs before the broadcast that it would “not have a significant impact or fallout” but that after the revelations the company had called “a crisis meeting” as it faced questions “both from within Drax and externally”.

His statement said the team in the UK “dealing with the fallout” were “not very connected with the team on the ground in Canada”, meaning that it had been a “challenge” to collect all the information needed to respond in a public statement and to the government.

“From very early on in our response to the Panorama programme it became clear to me that we needed to carry out a full review of the allegations made,” Gardiner said. His statement denied Ahmar’s suggestion that he had resisted a review over fears of what it might uncover.

In a witness statement, Jonathan Oates, director of external affairs at Drax, said the focus after the BBC documentary had been “on getting correct information out there in rebuttal” to the allegations. “The number one rule of public relations is that you do not lie,” Oates said.

Clare Harbord, the company’s former corporate affairs director, told the court that all the external statements made by the company had been “signed off by numerous people across the business” and that the “amount of cross-checking was extraordinary, meticulous and robust”.

The consultancy KPMG was tasked with reviewing the data Drax had provided to the regulator and the public statements made after the broadcast almost a month after the allegations emerged. Drax has declined to make the findings of the review public.

Ahmar left Drax in January 2024 after a period of “special leave” from her role. She reached a settlement with the company over the employment tribunal claim last year. Drax said the company and Ahmar had “reached a mutually agreeable position, without admission of liability”.

A spokesperson for Drax said the claims made against the company in the tribunal were “thoroughly and seriously investigated through our internal processes, an independent report by a leading employment KC and separate reports by a third-party on our biomass sourcing”.

The spokesperson added: “We provided all relevant material from this investigation to our regulator, Ofgem. Following their separate investigation, which ended in August 2024, Ofgem concluded that they did not find any evidence that we had been issued with [subsidies] incorrectly or that our biomass does not meet the government’s sustainability threshold. Ofgem also found no evidence of deliberate misreporting.”

The 16-month Ofgem investigation, which was not focused on the issuing of subsidies to Drax, found that there had been “an absence of adequate data governance and controls in place” when it came to profiling the sources of wood used by the company from Canada between April 2021 and the end of March 2022. Drax agreed to pay £25m in compensation for the breach.

A separate investigation by the Financial Conduct Authority continues. It is investigating “historical statements” made by Drax about the sourcing of pellets.

Drax – which produced 10% of Great Britain’s electricity in 2024 – was once western Europe’s largest coal-fired power station but committed to replacing coal with compressed wood biomass pellets in 2012 and completed the shift in 2023.