UK grid operator denies blackout risks during extreme heat event

Serious allegations over the security of Great Britain’s electricity grid have emerged in the wake of grid operator NESO issuing its first ever summer Electricity Margin Notice (EMN) on June 23.

On July 7 former UK Energy Secretary Claire Coutinho MP raised the “risk of blackouts from the increasing instability of our electricity grid” in a parliamentary debate, and informed the UK House of Commons that she had been contacted by a whistleblower “at the heart of our grid operator.”

“They allege that senior managers have ordered control room operators to hide information that shows that the grid is not being run securely. They also allege that corporate affairs have interfered with the operational decisions needed to stabilize the grid in order to protect the NESO’s reputation.”

The allegations, strongly denied by the grid operator, are focused on the week commencing June 22, when the United Kingdom and much of Europe experienced extreme heat, with high temperatures placing significant stress on power systems and creating tight operational margins.

The grid operator acknowledged that “unprecedented weather” placed significant stress on power systems in Great Britain, when a combination of low wind generation, reduced availability of some gas-fired generation, high sustained electricity demand, adverse interconnector flows and network constraints all created a significant operational challenge

In response, NESO issued its first ever summer EMN on the evening of June 23, warning of a system margin shortfall for the following evening. In a statement issued at the time, NESO described the margin notice as a “routine tool” used to ask market participants to make any additional generation capacity they have available. The grid operator added that an EMN does not mean the electricity supply is at risk.

NESO has since reported that the electricity system remained secure throughout the period in question, with the system frequency remaining within its statutory limits of 49.5 Hz to 50.5 Hz. The grid operator has also published the frequency trace covering June 22 to June 26, alongside explanatory notes from the director of system operations describing how frequency was managed in the control room.

Conservative Party energy spokesperson Coutinho has called NESO’s competence into question, informing the UK House of Commons on July 7 that she had been contacted by whistleblowers who allege the electricity grid is not being run securely, with particular concerns around the June 23 event.

Coutinho, who served as the Secretary of State for the Department of Energy Security and Net Zero (DESNZ) from August 2023 to July 2024, then wrote to NESO calling for an external investigation into the incident that should include anonymous interviews with control room engineers.

Coutinho also said she has been approached by more whistleblowers since raising the issue in parliament and she encouraged any other concerned NESO staff to reach out to energy regulator Ofgem, or directly to a member of parliament (MP), citing whistleblower protections in UK employment law.

NESO denies there was a risk to system security but confirmed it will launch an internal investigation due to the seriousness of the allegations. The investigation will be run by an independent external legal firm reporting to an independent non-executive member of NESO’s Board of Directors. Energy regulator Ofgem has stated that as NESO’s regulator, it will “make sure the review is transparent, impartial and has the right terms of reference to get to the facts about what happened.”

“During an unprecedented period of extreme heat and tight margins across Great Britain and Europe, the electricity system operated securely. No customer demand was disconnected, frequency and voltage remained within statutory limits, and no lines or cables were overloaded,” a NESO spokesperson said in a statement. “A full deep dive into the analysis of system and market operation is underway and will be published. This is routine after all such events. NESO has commissioned an independent investigation into recent allegations around decision-making and record-keeping. Its conclusions will be published.”

Multiple EMN notices have been issued since the June 23 alert, as the United Kingdom has continued to experience higher than average temperatures. The debate on the security of Great Britain’s energy system coincides with the publication of the UK Met Office’s latest annual State of the publication of the UK Climate report, which found 2025 was the warmest year since records began in 1884 – with the last four years in the top five warmest.