Imagine this. At 8.37pm tonight, as one section of the national grid is hit by a sharp imbalance between electricity supply and demand, the entire network starts to go down. One piece topples the next, like dominoes.

The country goes dark. The lights switch off in every home and business. Airports, hospitals and other essential utilities scramble to switch to back-up generators.

Trains and lifts judder to a halt. Mobile phone transmitter towers switch to battery and back-up power, but networks rapidly become congested and coverage progressively deteriorates.

Firefighters struggling to contain wildfires across the country face catastrophic disruption as computer systems and terrestrial communications fail or become overloaded.

Emergency service radio networks and satellite phones continue to function at first, but communications become increasingly difficult as back-up systems are stretched, batteries run down and generators exhaust their fuel.

With the blazes spreading, the services find it harder to establish which towns and villages are most in danger. Orderly evacuation becomes impossible, with confusion and panic spreading in the worst-affected areas.

By 10pm, with millions of homes in complete darkness, the risk of house fires begins to rise as people turn to candles, camping stoves, barbecues, portable generators and improvised methods of cooking and lighting.

Because fire crews are already stretched beyond capacity, some fires cannot be reached quickly and spread rapidly.

Wind turbines stand next to a solar farm near the Anglesey village of Rhosgoch in Wales

Road accidents increase as traffic lights, street lighting and electronic road signs fail. Police and safety crews are slower to respond because communications are degraded and roads obstructed, meaning some minor incidents develop into more serious collisions and create congestion.

With roads blocked and petrol stations unable to supply fuel because the pumps need electricity, thousands of drivers are stranded, spending the night in their cars or at the roadside, many without water, unable to tell loved ones what is happening.

By morning, the streets are crowded with people demanding to know what’s going on. The absence of internet, wifi and reliable phone signals stokes the confusion. Sporadic rioting breaks out, forcing the government to consider deploying the Army.

And this is just the first 12 hours.

Recent warnings from whistleblowers inside the National Energy System Operator (Neso) – which is responsible for the operation of the national grid – suggest that restoring the system after a total shutdown like this would be neither quick nor straightforward.

Although parts of the country might be ‘re-energised’ quickly, a complete restoration could take days, with some areas potentially without power for much longer.

Of course, all this hasn’t happened – yet. But the truth is that the national grid is more fragile than at any time since privatisation in 1990.

On Monday, it emerged that Neso would like to operate the grid with reduced levels of ‘inertia’, the kinetic energy stored in the spinning components of generators that carry on revolving even in the event of them losing power.

This move is designed to save £96million a year and help the government hit its Net Zero targets. But I consider this to be a very dangerous proposal as it will increase the possibility of blackouts – and I wrote to Ofgem last year to object to it.

The national grid is already under strain thanks to the ongoing heatwave. It was last month forced to halt exports to certain European neighbours via four of its ten interconnector cables for several hours.

This incident followed an even more serious event the month before. The gravity of the situation was highlighted by Conservative shadow energy secretary Claire Coutinho, who told MPs in early July that whistleblowers claimed Britain had narrowly avoided a ‘blackout’ on June 23 as temperatures reached 34C (93F).

Kathryn Porter, an energy consultant and founder of Watt-Logic, says she wrote to Ofgem last year to oppose the plan to operate the grid with reduced levels of ‘inertia’

Dawn breaks over a dark London skyline in 2021 – Kathryn says if millions of homes were in complete darkness due to a blackout, ‘the risk of house fires’ would rise

No one in a senior post at Neso wants to admit the scale of the looming crisis. Doing so would invite hard questions about senior management decisions and the adequacy of the systems on which the country relies.

Thanks to successive governments’ obsession with Net Zero, Britain’s approach to power generation has been transformed out of all recognition in a few short years.

When the Climate Change Act was passed in 2008, around four-fifths of our electricity came from fossil fuels. Fast forward 18 years and we have no coal-fired power stations at all, with the closure of the final plant – Ratcliffe-on-Soar Power Station in Nottinghamshire – bringing to an end 140 years of coal-fired electricity generation on September 30, 2024.

But while renewables are now responsible for 47 per cent of our domestic electricity production, gas remains a vital power source as it is impossible to predict with any great accuracy when the sun will shine and the wind will blow.

Indeed, Neso missed its target of running the electricity network for at least 30 minutes in 2025 without any fossil fuels. Yes, you read that correctly: renewables are incapable of meeting our power needs for even half an hour in a single year.

Our supply difficulties were made worse last year by the fact that, after years of decline, there was a 1 per cent rise in UK electricity demand in 2025, driven by cold weather and some recovery from the demand destruction caused by the 2022-23 energy crisis.

We also import far more of our electricity than at any time in our history – 14 per cent in 2025 compared with around 1 per cent in 2008.

And therein lies the problem. Our biggest sources of imported electricity are two countries experiencing generation problems of their own.

France, which has more nuclear power stations than any other European country, faces shutdowns due to age-related defects, with the French nuclear regulator warning of further risks as time goes on.

Meanwhile, Norway – which relies on a network of 1,100 reservoirs and 1,700 hydroelectric power stations to produce 90 per cent of its power – is suffering from a shortage of water.

As a result, both may be forced to restrict exports.

Electricity pylon is surrounded by dark clouds – Britain’s towns and cities are linked by 4,500 miles of overhead power lines supported by pylons

The other countries which export to us rely, as we do, on wind and solar and – as we share similar weather – can experience shortages when we do. With low European gas stocks heading into this winter, there are real concerns about the availability of electricity for export in these countries.

When Claire Coutinho attempted to raise the alarm in Parliament last month, the then Energy Secretary Ed Miliband accused her of scaremongering.

The reality is that, if anything, she was understating the peril we face. It’s no exaggeration to say our electricity system increasingly runs on hope and blind optimism.

This is because Neso’s engineers have no way of knowing how much solar energy is entering the system at any one time.

Britain’s towns and cities are linked by 4,500 miles of overhead power lines supported by pylons and around 900 miles of underground cables, and this high-voltage grid moves electricity at tens or hundreds of thousands of volts.

It’s a highly efficient way of sending electricity across country but impractical and dangerous for use in the home, where most appliances require a 230-volt supply.

And so the power first passes through a series of substations where transformers reduce the voltage to standard household levels.

However, electricity from most solar panels flows directly into the low voltage grid that feeds homes and businesses, which makes it very difficult for Neso engineers – who operate the high-voltage grid – to plan around it.

In effect, they have to guess how much is entering the system. To make matters worse, the software the engineers use to balance the grid was written back in the 1980s and is increasingly unreliable.

Just this week the entire balancing software failed, forcing engineers to revert to the use of spreadsheets and telephone instructions to hundreds of power stations in order to keep the grid running.

Because, absurdly, the software they use to balance the grid was written back in the 1980s.

The Houses of Parliament in central London sit in darkness after the lights were switched off for Earth Hour on March 29, 2014

To the casual eye it looks more like Ceefax, the BBC’s long-discontinued precursor to online news, than a modern operating system for a crucial national resource.

Unable to see where demand is rising or falling, they are – as one whistleblower told The Mail on Sunday recently – ‘flying blind’.

It also emerged this week that hundreds of power stations had their meters installed backwards, so they look as if they are using electricity instead of generating it. That Neso didn’t identify these problems during commissioning testing is astonishing.

In a fundamentally flawed system, this Heath Robinson approach goes only so far. Sometimes more drastic measures are called for. And on June 23 this year, as we have seen, the grid reached a crisis point.

Leaked reports from Neso insiders revealed that, as the summer heatwave pushed temperatures above 34C (93F), a spike in demand to power fans and air conditioning systems created a perfect storm.

As solar generation dropped steeply in the early evening and wind generation slumped to near zero, several gas-fired power stations suffered unexpected outages.

This led to grid conditions that exceeded safe operating limits for extended periods and NESO went into panic mode.

At the time, it was exporting electricity to the Netherlands through its cross-border cable but, in its desperation to bring the grid back into balance, Neso fell back on emergency protocols and abruptly halted those exports.

This was done without warning and without the prior agreement of the Dutch operator TeneT. And it still wasn’t enough to rectify the problem.

And so Neso decided it needed to stop sending electricity to the French, too. Given France’s status as a net exporter of power to this country, however, they had the sense to ask nicely before flicking the ‘off’ switch.

Such disruptions of service are far from ideal in a world where the exchange of energy between nations is vital to maintain a constant supply of electricity.

All this indicates that Neso didn’t see this crisis coming. It’s clear that they weren’t prepared for a shortfall of such proportions until it hit them – and then they started making increasingly drastic decisions.

Incredibly, inside sources say these decisions were influenced by Neso’s corporate affairs team, which was apparently keen to avoid any grid-stabilising actions that might involve rationing, with all the reputational damage that would involve.

Neso has since confirmed that its PR team was present in the control room as the events of June 23 unfolded but says it was just one source of ‘input’ into operational decisions.

Despite the accounts given by whistleblowers, Neso now claims there was no real crisis on June 23. Such a reaction does not instil confidence. If Neso flatly refuses to admit the problem exists, it’s bound to re-occur.

And next time we might not be so lucky. Nationwide blackouts could become a familiar reality.

Two of our European neighbours know just how calamitous a full electricity shutdown can be. In April last year, both Spain and Portugal lost power, with the outage lasting for 16 hours in some regions.

Officially, the incident caused 11 deaths, including several people who suffocated after their mechanical ventilators shut off.

Three other victims died from carbon-monoxide poisoning as faulty generators pumped out the lethal gas, and one person perished in a house fire caused by a candle.

However, a subsequent study estimated that the toll was far higher, with 165 ‘excess deaths’ over two days attributed to the blackout, most likely from falls, delays to treatment and other causes.

The European regulator ENTSO-E described it as ‘the most severe and unprecedented blackout that had occurred in Europe in the past 20 years’.

Fortunately, the blackout occurred during benign weather. Many more people could have died if it had happened in the middle of winter or during a heatwave because – without electricity – heating and cooling systems generally won’t function.

And a resumption of power supply is not straightforward. In Britain, getting the lights back on once power is restored is a major undertaking, as it involves using a seat-of-the-pants method known as a ‘black start’ that has never been fully tested.

When a power station has been offline for maintenance, it is usually restarted using power from the national grid. But this isn’t an option when the whole grid is down.

In a black-start situation, some large power stations use diesel generators to get going. They then have to trickle just enough power to the grid to energise other power stations until enough generation is available on the high-voltage grid to meet expected demand on the low-voltage grids.

But if that demand is not estimated correctly, not only would the grid immediately trip off, but some equipment could be overloaded and break.

For that to happen after the country has already been in a nationwide blackout for days would be catastrophic.

Little wonder that the mood among national grid engineers is close to despair. In a letter to The Mail on Sunday last month they literally begged the public for help in persuading Ofgem to investigate their fears over grid security.

Given that almost every aspect of modern life is reliant on electricity, maintaining a safe, robust national grid ought to be a top priority. Instead, we have become dangerously complacent.

Britain is living with an unimaginable risk, yet most people know nothing about it, and the government doesn’t seem to want to know.

When we are finally forced to face reality, it will be too late.

Kathryn Porter is an energy consultant and founder of Watt-Logic