Details of tens of millions of voters could have been accessed by hackers who targeted the elections watchdog.
The Electoral Commission said there was little risk of “hostile actors” being able to influence the outcome of a vote, but apologised for the breach in its systems.
The hack, which was publicly confirmed on Tuesday, allowed the attackers to access reference copies of electoral registers containing the names and addresses of people registered to vote between 2014 and 2022.
The attack was identified in October 2022, but the hackers had first been able to access the commission’s systems in August 2021.
Shaun McNally, the Electoral Commission’s chief executive, said: “The UK’s democratic process is significantly dispersed and key aspects of it remain based on paper documentation and counting.
“This means it would be very hard to use a cyber-attack to influence the process.
“Nevertheless, the successful attack on the Electoral Commission highlights that organisations involved in elections remain a target, and need to remain vigilant to the risks to processes around our elections.”
He said significant measures had been taken to improve security on the commission’s IT systems.
“We know which systems were accessible to the hostile actors, but are not able to know conclusively what files may or may not have been accessed,” he said.
“While the data contained in the electoral registers is limited, and much of it is already in the public domain, we understand the concern that may have been caused by the registers potentially being accessed and apologise to those affected.”
The hackers were able to access reference copies of the electoral registers, held by the commission for research purposes and to enable permissibility checks on political donations.
The registers held at the time of the cyber-attack include the name and address of anyone in the UK who was registered to vote between 2014 and 2022, as well as the names of those registered as overseas voters.
But they did not include the details of those registered anonymously.
The register for each year holds the details of around 40 million individuals, which were accessible to the hostile actors, although this includes people on the open registers, whose information is already in the public domain.