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In a control room in Wokingham, Berkshire, staff work around the clock, minute by minute, to align Great Britain’s fluctuating power supplies with its ever-changing demand.
However, as households struggle to adapt to soaring summer temperatures, staff at the government-owned National Energy System Operator (Neso), tasked with keeping the lights on, have faced the kind of challenge usually reserved for the country’s coldest winter nights in recent weeks.
The high pressure “heat dome” that descended on Europe in late June caused wind turbines to slow, gas power plants to falter, and the French nuclear plants exporting electricity to Britain to power down. Meanwhile, electricity demand for air conditioning and refrigeration units jumped as Britons attempted to stay cool.
The task of keeping the grid’s electricity supplies stable during the June and July heatwaves was set against the backdrop of a political row, and a culture war over Britain’s ambitious clean energy targets and the cost of reaching net zero.
Earlier this month the shadow energy secretary, Claire Coutinho, told parliament that whistleblowers from within Neso had accused senior managers of being involved in a cover-up over the extent of the power system strain during record high temperatures in the UK and mainland Europe in late June.
The accusations included the claim that control room staff were ordered by senior managers to forgo normal note-keeping to avoid creating a full audit trail of their efforts to stabilise the power system. Whistleblowers have also alleged that members of the corporate affairs team attempted to pressure operators to make decisions that would protect the organisation’s reputation, Coutinho said.
The employees came forward because they were “worried that the grid is becoming increasingly unmanageable” and that senior bosses were “risking blackouts to protect Neso’s reputation”, she said. “If true, this is nothing short of a scandal.”
Last week, Neso said that it had brought in independent investigators from the law firm Eversheds Sutherland to examine the accusations.
So what do we know about how the system operator handled the heatwave? The whistleblower allegations focus on the last week of June. It was a week that demonstrated that summer heatwaves can “now create system conditions traditionally associated with winter scarcity events”, according to LCP Delta, a consultancy.
During that week, Neso issued a string of emergency measures to shore up Great Britain’s electricity supplies, marking the first time these tools were used during a summer period. To underline the challenge, figures this week showed Neso spent almost £800,000 getting consumers to use less electricity than normal between mid-April and mid-July, according to the Financial Times.
Despite these levers, the frequency of the grid – which Neso aims to keep at about 50Hz – fell to the edge of its operational limits of between 49.8Hz and 50.2Hz.
The operation range, which is used to prevent blackouts, remained under pressure for the better part of an hour on 23 June. The lowest recorded frequency was 49.66Hz, still well above the statutory lower limit of 49.5Hz.
However, a former employee told the Guardian that the prolonged period of lower than usual frequency may suggest that the system operator was running low on politically palatable options to address the strain on the grid before more serious interventions, such as disconnecting factories, might have been needed.
“When I saw the grid frequency data I thought it just couldn’t be right because it was at the lower end of the operational limits for such a long time. A 0.2Hz change might not seem very large but in technical terms it represents a much greater vulnerability. At these levels there is a risk that something unexpected might happen which the system would be too strained to absorb,” they said.
Neso has downplayed the concerns. Although the heatwave led to “sustained pressure on power systems” and “tight operating margins”, according to Craig Dyke, a Neso director, “no customer demand was disconnected, frequency remained within statutory limits, the voltage stayed within limits and no lines or cables were overloaded”.
In two instances in the last week of June, the system operator was forced to raise the alarm over electricity supplies because of the impact of the heatwave, which scientists believe was more severe and more likely happen because of the climate crisis.
This resulted in payments estimated to reach millions of pounds for just a few hours of emergency electricity generated by gas power plants, a leading cause of the atmospheric carbon emissions that are accelerating global heating.
Neso controllers are also understood to have breached the normal limits on trading power across the giant subsea power cables linking Great Britain’s electricity system to generators in continental Europe “several times” across the week, according to analysts.
The largest of these breaches occurred on 24 June, when Neso issued an emergency call for an additional 500MW of power capacity from the Netherlands through the BritNed power line, achieved by cutting Great Britain’s exports. A similar emergency call was made to France.
Some market experts are wary about any suggestion that Great Britain may have experienced a blackout “near miss” during the heatwave. Blackouts are extremely rare in the UK, and when they do occur they are the result of a confluence of unforeseen events.
In 2025, Europe’s worst blackout in 20 years – across Spain, Portugal and parts of southern France – was blamed on multiple interacting factors that destabilised the grid’s frequency. In 2019 there was a short-lived blackout in the UK when a lightning strike caused two electricity generators more than 100 miles apart to disconnect from the grid in an “extremely rare and unexpected event”.
The risk to Neso’s reputation from a potential near miss is significant, the former employee stressed. In a worst-case scenario, Neso can resort to lowering the grid’s voltage to prevent a widespread blackout or even disconnect large industrial energy users to keep the lights on in homes across the country. “But that’s what you’re hoping you never have to do. That’s the last thing you want,” they said.
The pressure to avoid reputational damage would be nothing new for those in the Wokingham control room. Neso became a state-controlled body after the government bought it from the FTSE 100 energy company National Grid in late 2024 for £630m.
Under its previous ownership, engineers were aware a misstep could hit National Grid’s share price; now they face the weight of expectation to balance supply and demand without a hiccup as more clean energy sources come on to the grid and Labour pushes to hit a 2030 green energy target, the former control room worker said.
“Whether there were direct orders to cover up a scandal or not, individuals are potentially being put in an invidious position that might inhibit them from taking the correct action. There was always that sense of being wary of how your actions might be viewed by those higher up. This might mean not taking timely actions [to rebalance the system] if you are fearful that it might cause senior managers to look askance,” they added.
In a letter to the industry regulator for Great Britain, Ofgem, the Neso chief executive, Fintan Slye, said the system operator had “policies and procedures in place” to ensure that this kind of influence could not take place. “Nevertheless, we take these allegations extremely seriously,” he said. A spokesperson said the Eversheds investigation will be shared with Ofgem and the government, and ultimately published. It is unclear exactly when that will be.
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