Could a Winter Blackout Freeze the UK’s Financial Services?
Illustration by James Daw
Key takeaways
A UK blackout lasting more than 24 hours before 2026 was estimated at 7.8% – a non-negligible risk with serious consequences for financial services
The likelihood rises significantly in a severe winter, when cold, storms, and tight gas supplies could push the probability to 14.8%.
If such a blackout occurs, forecasters see better than even odds that at least 100,000 people would lose access to their bank accounts, with cascading failures across ATMs, payments, and telecoms.
In April, a widespread power outage in Spain showed how quickly a nation’s energy infrastructure can be stressed. If a similar outage were to occur in the UK and stretch beyond 24 hours, the effects would escalate from inconvenience to systemic risk for financial services: payment flows could seize up, trading could be disrupted, and millions could lose access to banking services. For a sector built on real-time settlement and continuous connectivity, this is a severe but plausible scenario that deserves active monitoring.
These vulnerabilities are amplified by mounting pressures on the UK’s power system. Electricity demand is climbing, driven in part by the growth of data centres, the electrification of transport and heating. Much of the grid infrastructure is aging, with uneven investment in upgrades. Winter peaks are increasingly shaped by extreme weather, and longer spells of low wind or sun can tighten operating margins just as demand surges. Findings
Against this backdrop, the Swift Centre Forecasters were asked: Will the UK experience an electricity blackout lasting over 24 hours before December 31st 2026 that severely disrupts financial services?
Findings
On average, the Swift Centre forecasters put the probability of the UK experiencing a blackout lasting more than 24 hours before the end of 2026 at 7.8%. This means roughly a one-in-twelve chance of a systemic disruption to the UK’s financial services within the next two years. And this figure only covers the most severe case – outages lasting more than 24 hours. Shorter blackouts, which can still disrupt payments and trading, are supposedly more plausible.
Behind this headline number, views diverged. A group clustered at the higher end, around 17-18%, emphasising severe weather and infrastructure fragility. Others leaned much lower (0.5-5%), pointing to the historical rarity of such long outages and the grid’s proven resilience. The gap reflects different judgments about what could plausibly drive a multi-day blackout, as well as competing views on the UK’s ability to withstand shocks.
Those on the lower end argued that blackouts lasting more than 24 hours are historically rare in the UK and that the grid has generally proven resilient. One noted: “We have events that can be considered ‘major’ UK blackouts… However, few if any lasted longer than 12 hours.” Another gave a 2% estimate but said this would rise to 5-10% if shorter outages were considered. In their view, the most plausible drivers of a multi-day blackout would be supply shortages rather than extreme weather. While storms and floods regularly cause localised outages, these are usually resolved in under 24 hours. By contrast, past rolling blackouts were linked to fuel constraints, and the UK’s relatively small gas storage capacity still represents a vulnerability:
“We currently store 12 days’ worth of winter gas demand, compared with 89 days in Germany and 103 days in France.”
Even so, they judged that interconnectors to Europe and imports from the U.S. and Middle East make outright shortages unlikely in the forecast period. Cyberattacks were also downplayed as a plausible trigger. The grid remains relatively isolated from the internet, and past incidents (such as the 2019 lightning strike) have been rapidly contained:
“A long, nationwide blackout won’t occur, in spite of the Grid’s finer supply and response time margins.”
Others in the low-probability camp nudged their estimates slightly higher (up to 5%), emphasising recent close calls and the challenges of managing a system with more variable renewables. One observed that “risk is higher than in the past with increased share of volatile renewables.” Another pointed to the near-blackout in January 2025: “Such close calls do make one wonder if the system is sufficiently resilient. But at the same time, that the situation at worst would have resulted in planned blackouts of about three hours.”
By contrast, those at the higher end placed more weight on weather and infrastructure fragility. One reasoned that “blackouts in the UK are not particularly frequent and almost all of them […] are related to severe weather,” and identified 3-6 instances in the past three decades where outages lasted over 24 hours, leading them to a double digit estimate. Another highlighted specific cases – the 2013 Christmas storms, 2015 floods, and storms Arwen (2021) and Eunice (2022) – and concluded that “large towns going out for more than 24 hours is quite common.” A third, giving a 17% probability, focused on infrastructure risk, pointing to Heathrow’s 18-hour outage in 2025 as a sign of weak redundancy:
“The recent shutdown of Heathrow due to a substation fire showed a weakness in ability to quickly shift to alternative sources.”
They also added a small premium for cyberattacks, accounting for around 1.5% of their estimate.
If Winter Bites Harder
Forecasters were also asked to assess how their probabilities would change in the event of a severe winter in 2025 or 2026. In this scenario, severe winter was defined as record-low temperatures, low renewable generation, tight gas supplies, or extreme storms.
Across the board, forecasters agreed that harsh winter conditions would make a long blackout more likely. Most described the effect as roughly doubling their baseline estimate, though the results varied widely depending on where they had started from. Justifications also differed: some emphasised the compounded stress on gas and renewables, others the physical damage caused by storms, and a few highlighted the risk of global energy supply constraints.
For some, the increase was modest in absolute terms. One forecaster, for example, raised their probability from 2% to 4%, explaining:
“Even given a severe winter, I think the probability is very low as long as parts of the large town or region have power restored in under 24 hours, so I'm increasing my estimate by only two percentage points.”
Another echoed this view, stressing that “Individual weather events like blizzards can cause blackouts without mounting to an extreme winter, so I go with a moderate increase.”
Others arrived at higher final estimates. One calculated that roughly “47% of years with a major outage… would have resolved positively” under the severe winter condition, leading them to double their baseline estimate to 40%. Another, who considered the combined risks of high winds, low temperature, major snow or ice storm, argued that resilience would be lower in such circumstances:
“Even the loss of a couple of power stations would be a major concern to the UK’s power grid.”
They concluded that the probability would be about double their baseline, at 9.8%.
Several forecasters pointed to infrastructure fragility and gas reliance as the key vulnerabilities exposed by severe winter conditions. One noted:
“The UK increasingly relies on natural gas and renewables. Given such reliance, a period of extreme cold along with low winds could [cause] selective power outages.”
Another cited National Energy System Operator projections showing “the strongest electricity operational margins for the UK grid since 2019,” but still allowed for the possibility that an unusually cold winter could coincide with global demand pressures and LNG import shortages, settling on a 20% estimate.
A final forecaster placed their probability at 25%, reasoning that while severe weather is rare in the UK, it can be very difficult to repair quickly when it does occur:
“Ice storms are very rare in the UK, but if they occur, can be devastating to power lines and time-consuming to repair. … I expect that loss of power to a medium-sized city for 24 hours is a relatively low bar and would be difficult to solve without repairs of numerous transmission lines.”
Knock-On Effects on Banking Services
Forecasters were also asked to estimate the likelihood that such a blackout would prevent at least 100,000 people in the UK from accessing their bank accounts before 2027.
All forecasters judged this outcome to be plausible, with estimates ranging from 30% to 75% and averaging 58%. That implies better than even odds that a major blackout would leave more than 100,000 people unable to access their bank accounts. While confidence levels varied, the consensus was clear: a prolonged outage would almost certainly disrupt financial services on a large scale.
At the top end, one forecaster assigned a 75% probability, arguing that the scale of disruption in a 24-hour+ blackout would inevitably capture this many people. Others supported this view with slightly lower estimates. One (62%) explained:
“If it lasts for more than 24 hours, it’s likely a nationwide blackout… This very likely means that more than 100,000 people would be unable to access their bank accounts or make payments for at least 12 hours.”
Another (63%) stressed that “branches and ATMs would naturally be down… The last break point could be the one that matters the most, since an increasing number of people rely on the internet to bank.”
Several other forecasters converging around the 50-60% range, emphasised that mundane banking services would fail under these conditions. One wrote:
“Should a blackout happen, just about everyone affected by a blackout will lose online access to bank accounts. The main banks may function OK for some time but [the] majority of the cashpoints will be down. And of course 99% of stores of all kinds will not be able to receive card payments.”
Another added that “even for those with access to banking by phone app and laptop, cell tower operation, internet service, and the ability to recharge devices are likely to put the number over 100,000.”
The only forecaster at the lower end (30%) argued that the internet is not always disabled during power outages, and that people might continue using phones and other battery-operated devices, limiting the number directly affected.
Despite these differences, the consensus was clear: once a 24-hours blackout occurs, cascading failures across ATMs, payment systems, telecommunications, and local infrastructure make it highly likely that at least 100,000 people will lose access to their bank accounts.
Conclusion
Forecasters judged a multi-day UK blackout to be relatively unlikely – in the single digits under normal conditions – yet the aggregated estimate of 7.8% (with highs of 18%) is still concerning. For a risk that could paralyse payments, trading, and customer access to banking services, even single-digit probabilities amount to a material threat. The risk also rises further under severe winter conditions, with several forecasters effectively doubling their baseline estimates in that scenario. And the consensus was clear: if such a blackout were to occur, the impact on the financial system would be severe.
