The Mirror reports that hackers linked to Russia have targeted Zaun, a firm that handles security for some of Britain's most secretive sites, including a nuclear submarine base and a chemical weapon lab in Porton Down.
The stolen information was then placed on the dark web.
The Mirror reports that the leaked data could potentially help criminals gain access to the top-secret MoD sites.
The attack on Zaun occurred last month, and LockBit, the hacking group responsible, is considered to be the world's most dangerous hacking gang.
Mikhail Matveev, a key suspect in the gang, is on the FBI's Most Wanted list.
LockBit has previously attacked 1,400 global targets and attempted a £66 million blackmail on the Royal Mail.
The group is believed to have financial links to Russian gangsters.
Some of the leaked documents contained detailed information about security equipment purchased by Zaun to protect sites like Porton Down and HMNB Clyde.
The leaked documents also include sales order reports for equipment used at other sensitive sites like the GCHQ communications complex in Bude, Cornwall, RAF Waddington, and Cawdor Barracks.
As well as this, the stolen information includes documents relating to high-security prisons, such as Lon Lartin and Whitemoor.
Specific details about the equipment have not been published, and Zaun has not disclosed information about any ransom demands.
Labour MP Kevan Jones, who sits on the Commons Defence Select Committee, has expressed concern over the potential damage to national security.
"This is potentially very damaging to the security of some of our most sensitive sites."
A security expert described the incident as a devastating blow to national security infrastructure.
LockBit has issued £80 million in ransom demands globally and has been on the radar of the FBI since 2020.
Russian nationals Rlan Magomedovich Astamirov and Mikhail Vasiliev have been charged in the US in connection with LockBit ransomware attacks.
Zaun has reported the cyber attack to the police and claims that no classified documents were compromised.
The Mirror reports that The Government has declined to comment on security matters.
Security and intelligence expert Professor Anthony Glees has emphasized the significance of the leaked information, stating that every detail helps hostile actors breach the UK's defences.
He also pointed out that sloppy protocols, particularly by suppliers, pose a weakness in national security.