The government suspects China was behind the hack of an armed forces payroll system, the BBC understands.
Defence Secretary Grant Shapps will not identify a specific culprit when he addresses MPs today, but is expected to warn of the dangers posed by cyber espionage from hostile states.
The system used by the Ministry of Defence (MoD) includes names and bank details of armed forces personnel.
In a very small number of cases, the data may include personal addresses.
The system was managed by an external contractor.
The government became aware of the data breach in recent days, and has not found evidence hackers removed data from the system but is acting as if they did.
Sources have told BBC News the investigation into who was behind the breach, which will be seen as embarrassing for the MoD, is at an early stage.
It can take months, sometimes years, to gather enough evidence to publicly accuse so China is unlikely to be officially named today.
However, that does seem to be where suspicions are pointing towards, especially in light of Beijing’s track record of targeting these kind of data sets.
Service people affected by the hack will receive further information from the government about the breach and will be told any concerns are more about fraud risks rather than personal safety.
Source: BBC
