Email addresses and encrypted passwords of British military, intelligence and police officials were published online following a hack attack, according to an analysis of the data.
The actual compromise took place late last year but a Guardian analysis of the information has revealed the quality of data stolen.
Government defences were not broken, but a Texan consultancy named Stratfor, which held the data, was hit.
At present, there is no indication of any threat to UK government systems.
Usernames and encrypted passwords of around 850,000 users who had subscribed to Stratfor's website were taken. Credit card numbers and addresses were also exposed.
The hackers are believed to be part of Anonymous, the hacktivist group which wreaked havoc in 2011, attacking various corporations and public organisations.
Amongst the leaked data were email addresses of 221 Ministry of Defence officials and information relating to officials from various UK Government departments, including the Cabinet Office and the Foreign Office.
"We are aware that subscriber details for the Stratfor website have been published in the public domain," a Government spokesperson said.
"At present, there is no indication of any threat to UK government systems. Advice and guidance on such threats is issued to government departments through the Government Computer Emergency Response Team."
Hacktivist group Anonymous has continued enacting its controversial tactics in 2012, claiming another hit on electronics giant Sony last week.
Anonymous member s3rver_exe said on Twitter they were responsible for the hack, along with others.
"Remember Not only i hacked Sony Anonerd and N3m3515 did too," s3rver_exe said.
Sony reportedly fixed the website and its compromised Facebook page quickly.
It is believed Anonymous is targeting Sony for its support of the US Stop Online Piracy Act (SOPA).
