- Written by Alex Forsyth
- BBC News political correspondent
The BBC understands that a major data breach has accessed the personal details of an unknown number of active British military personnel.
The hack targeted the payroll system used by the Ministry of Defense, which contained the names and bank account details of current and some former military personnel.
In rare cases, the data may include an individual's address.
It is unclear who is behind the hack or what the data will be used for.
Described as 'HMRC-style personal information', this data relates to current and former members of the Royal Navy, Army and Royal Air Force over several years.
This system is managed by an external contractor and no operational MoD data is captured.
The MoD is understood to have taken immediate action and the system has been taken offline while an investigation takes place.
The Ministry of Defense is in the process of informing those affected and offering support and advice, including informing veterans' organizations of what has happened.
Defense Secretary Grant Shapps is due to update MPs on the House of Commons hack on Tuesday.
In response, he is expected to develop a “multifaceted plan” that includes actions to protect affected servicemen and women.
It is not clear who is behind the hack, but it comes amid growing warnings about the cybersecurity threats the UK faces from hostile countries and third parties.
Last year, the government released an update to its long-term defense strategy, which stated that “the use of commercial spyware, ransomware and offensive cyber capabilities by state and non-state actors is rapidly increasing.”
Public institutions and private companies are also being targeted by hackers demanding ransoms.
The Metropolitan Police Department says it is not involved in the investigation at this stage.