Britain’s electricity grid operator reversed a call for extra power generation on Wednesday evening as temperatures soared past 35C, marking the hottest June day on record.
The National Energy System Operator (Neso) had issued an electricity margin notice for between 7pm and 10pm, a plea for more power to meet demand as air conditioners and fans strained the network. But hours later, it withdrew the alert without explanation.
“Britain's grid operator cancelled a power warning during a record June heatwave as temperatures hit 35.1C.”
The U-turn came as the heatwave tightened its grip on the UK, with temperatures reaching 35.1C in parts of the country — a record for June. The red weather warning, described as a “risk to life”, had already forced schools and offices to close, while Wimbledon qualifying at Roehampton was suspended after the electronic line-calling system failed during a power outage.
Neso declined to comment on why it cancelled the margin notice, but the move suggests the immediate risk of blackouts had passed — or that additional capacity had been secured. The margin notice, designed to signal tight supply, is a rare step for the operator, which typically relies on market forces to balance the grid.
The heatwave, which has brought travel chaos and health warnings across London and the South East, is expected to persist into the weekend. Londoners have been advised to avoid using car air conditioning unnecessarily, as some local authorities can impose fines of up to £80 under anti-idling regulations.
Temperatures on Wednesday surpassed the previous June record of 35.1C set in 2023, according to provisional Met Office data. The extreme weather has also triggered a surge in electricity demand, with the grid under strain as the public turns to cooling systems. Neso’s initial alert, issued early on Wednesday, had warned that spare capacity could fall below the safety buffer. Its cancellation brought relief to businesses and households already grappling with the heat.
The episode underscores the challenges facing Britain’s energy network during extreme weather events, as climate change increases the frequency of heatwaves. While the operator avoided emergency measures this time, the record temperatures and the sudden reversal of the warning have raised questions about the resilience of the grid in a warming world.