The Ministry of Defence is investigating allegations that Russian cybercriminals stole and released hundreds of classified military files online.
The documents were published on the dark web, an unregulated hidden area of the internet that can only be accessed through special software and is mainly used by criminals for illegal purposes.
They contain classified details of eight RAF and Royal Navy bases, as well as MoD staff personal details and emails.
Among the compromised locations is RAF Lakenheath in Suffolk, home to American F-35 fighter aircraft, according to the Mail.
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The base has been operated by the US Air Force since 1948 and is the largest US Air Force base in the UK, providing combat airpower to the region.
The MoD confirmed it was "actively investigating the claims that information relating to the MoD has been published on the dark web".
It added: "To safeguard sensitive operational information, we will not comment any further on the details."
Maintenance and construction contractor Dodd Group confirmed it suffered a ransomware incident, and it was taking the claims "extremely seriously".
The Ministry of Defence is investigating allegations that Russian cybercriminals stole and released hundreds of classified military files online
|PA
A spokesman from the group said: "We can confirm that the Dodd Group recently experienced a ransomware incident whereby an unauthorised third-party gained temporary access to part of our internal systems.
"We took immediate steps to contain the incident, swiftly secure our systems and engaged a specialist IT forensic firm to investigate what happened.
"We are taking these claims extremely seriously and are working hard to validate this."
The MoD has been a victim of several high-profile data breaches in the last 24 months.
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Among the compromised locations is RAF Lakenheath in Suffolk, home to American F-35 fighter aircraft, according to the Mail
|GETTY
In August, the details of more than 100 Britons, including spies and special forces, were included in a huge data leak that resulted in thousands of Afghans being secretly relocated to the UK.
The dataset, containing the personal information of nearly 19,000 people who applied for the Afghan Relocations and Assistance Policy (Arap), was released "in error" in February 2022 by a defence official.
The Ministry of Defence (MoD) became aware of the breach more than a year later, when excerpts of the spreadsheet were anonymously posted in a Facebook group in August 2023.
Other details leaked included the names and contact details of the Arap applicants and the names of their family members.
The documents contained classified details of eight RAF and Royal Navy bases, as well as MoD staff personal details and emails
|GETTY
The information was revealed on Tuesday, July 15, after a High Court judge lifted a super-injunction.
In a separate breach last year, the personal information of an unknown number of serving UK military personnel was accessed in a significant data breach.
The hack targeted a payroll system used by the Ministry of Defence, which includes names and bank details of both current and some past armed forces members.
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