The Ministry of Defence said the laptop, containing personal information about people who had joined the Navy, Marines and Air Force and from people keen to join, was stolen from the officer in Birmingham.
The timing could not be worse for Brown, currently on a trade visit to China and India. He is seeking to put behind him several government blunders that have seen his popularity plummet in opinion polls.
In November, details on 25 million child benefit claimants were lost by the Revenue and Customs department and a week later information on three million learner drivers went missing.
Last month, it also was revealed that medical records had disappeared at some health trusts.
In the latest case, entries on the stolen computer included family and passport details which could be used by fraudsters bent on identity theft.
Police said the theft was being "thoroughly and professionally investigated" by their officers in conjunction with the Ministry of Defence.
The theft took place on January 9 but was not revealed until nine days later because of the possible impact that might have on the investigation, the MoD said.
Media reports of the security gaffe prompted them to change their mind.
In a statement, it said: "The stolen laptop contained personal information relating to some 600,000 people who have either expressed an interest in, or have joined, the Royal Navy, Royal Marines and the Royal Air Force.
The information held is not the same for every individual. In some cases, for casual inquiries, the record is no more than a name."
But it did concede that "for those who progressed as far as submitting an application to join the Forces, extensive personal data may be held, including passport details, National Insurance numbers, drivers' licence details, family details, doctors' addresses and National Health Service numbers".
Revealing such a wide array of information could have devastating consequences for anyone trying to defend themselves from identify theft.
Banks holding personal accounts that could have been affected by the security breach have been "flagged for scrutiny against unauthorised access".
