Independent investigators have found significant and concerning “cultural issues” at Great Britain’s grid operator after control room staff accused senior executives of covering up the extent of blackout risks in the UK during the June heatwave.

An external legal firm tasked with investigating a series of whistleblower allegations against the National Energy System Operator (Neso) raised concerns over how high levels stress among control room engineers was handled by the company.

The three-month investigation began after whistleblowers accused Neso of a cover-up over the growing risk of blackouts. They claimed the grid was becoming increasingly unmanageable, and that control room decisions were subject to “interference” by the company’s corporate affairs team to protect the reputation of the government-owned operator.

The unprecedented control room revolt included multiple internal complaints, the leak of internal documents to the press, and accusations of whistleblower intimidation.

Neso was forced to abandon plans for an internal investigation into the claims in favour of an independent investigation by the law firm Evershed Sutherlands, whose findings were published on Monday.

The law firm said that while it found “no evidence” of inappropriate influence over operational decisions, or that control room engineers were warned against leaving a paper trail of their efforts to stabilise the grid, the investigation found “cultural, governance and control issues” that were “significant findings in their own right”.

The allegations against Neso emerged after the June heatwave caused grid frequency levels to fall below normal operating limits on a number of occasions and required emergency assistance from neighbouring countries to keep the lights on.

The grid woes were caused by near-record temperatures that caused demand for electricity to run air-conditioning systems to surge while a high pressure heat dome slowed wind turbines and prompted unplanned outages at gas and nuclear plants across Europe.

Tim Jarvis, the chief executive of the regulator Ofgem, said that although the investigation did not uphold the whistleblower’s specific allegations, it raised “important and concerning issues about working culture and practices at Neso”.

The investigation found there were “high levels of stress and concern” within the control room over how staff would be able to keep the power system balanced as renewable energy replaces legacy fossil fuel power plants in line with the government’s net zero ambitions.

It said there was a view that senior Neso executives did not fully appreciate or understand the pressures faced by the control room engineers who were responsible for keeping the lights on “on the frontline”.

In addition, staff felt that their concerns or complaints went “largely unheard” because of the need to keep external stakeholders including the government and the regulators happy, according to the law firm’s report.

Fintan Slye, Neso’s chief executive, said the report was “constructive” in identifying areas in which the grid operator could improve how staff could raise concerns and how to respond to them.

“I want all colleagues at Neso to feel heard. We will act on the recommendations, working closely with our trade unions, and be open with colleagues about the progress we make,” he said.

A separate Ofgem investigation, which is ongoing, will look into whether the cultural issues identified by Eversheds played a role in the operational decision-making, system security or raising energy costs for consumers.

“We will not prejudge that assessment,” Jarvis said, “But nor will we shy away from its conclusions. Our job is to establish the facts, identify the lessons and ensure action is taken.”

The Ofgem findings, which will be published no later than 31 March 2027, could lead to an enforcement investigation by the regulator if it believes Neso has not complied with obligations under market rules.

Michael Shanks, the minister for energy, said the government expected “significant action without delay” and would “keep progress under close review”.