The four-day outage is believed to be one of the most significant cyber-attacks against UK energy infrastructure. Hackers linked to Iran shut down a small British power plant for four days in what is believed to be the first successful cyber-attack of its kind against a UK energy facility, according to The Telegraph. The incident is understood to have happened last month, at the same time as a series of cyber-attacks targeting water infrastructure across the US. The affected UK facility was relatively small and the outage had no impact on the wider electricity supply or Britain's overall energy generation, according to the report. British officials have declined to identify the plant, citing security concerns. The incident was reported to the National Cyber Security Centre (NCSC), the public-facing arm of the intelligence agency GCHQ responsible for helping protect UK organisations and critical infrastructure from cyber threats. While foreign hacking groups have previously disrupted NHS systems, schools and industrial facilities, no hacker is thought to have previously succeeded in bringing a British power plant to a standstill. The four-day outage is therefore believed to be one of the most significant cyber-attacks against UK energy infrastructure. The plant is understood to have been a small-scale facility. Britain has dozens of such generators connected to the national grid, many of which operate only for limited periods when additional electricity is needed. The disruption therefore did not threaten the wider grid. A UK government source described the incident as highly limited in scale. "We have thresholds for important generators to legally notify us of cyber activity, and this site is nowhere near. It’s a very small scale site, less than a rounding error compared to grid capacity," the source told The Telegraph. A government spokesman added: "The UK has a highly resilient energy system. We work closely with the energy sector to protect infrastructure and ensure the highest security standards. "This story refers to an incident impacting a small-scale energy generator, and at no point was there a risk to the wider energy system." The attack comes amid growing warnings about Iranian cyber activity against Western countries following the conflict in the Middle East. Iran has significantly increased its cyber operations since the US and Israel began air strikes in February, with suspected Iranian attacks reported in several European countries. The NCSC warned British organisations in March to review their cyber-security measures because of the conflict. Its chief executive, Richard Horne, said in June that the agency had dealt with more than 200 attacks against critical national infrastructure during the previous year. The NCSC does not routinely confirm individual cyber incidents and declined to comment on the power plant attack. The incident is also understood to have prompted the government to brief chief executives of British power companies and send advice to businesses on how to respond to the threat. The UK incident reportedly occurred around the same time as a series of attacks against water infrastructure in the US. The attacks affected wastewater treatment plants in several states, causing flooding and loss of water pressure. Some authorities advised residents to boil water before drinking it. The first reports emerged in Minnesota on 26 July, followed by incidents in Michigan, Georgia, South Dakota and New Jersey.