The personal information of an unknown number of serving UK military personnel has been accessed in a significant data breach, the BBC understands.
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.
In a very small number of cases, the data may include personal addresses.
It is unknown who is behind the hack or what the data may be used for.
The data, described as personal HMRC-style information, relates to current and former members of the Royal Navy, Army and Air Force.
The system was managed by an external contractor and no operational MoD data has been obtained.
It is understood the MoD has taken immediate action and the system has been taken off-line, while investigations are under way.
The MoD is in the process of notifying and providing support and advice to those affected, including making veterans' organisations aware of what has happened.
It is thought MPs could be informed about the development in the Commons on Tuesday.