British military officials, civil servants and parliamentarians have had their email addresses published on the internet after they were obtained by hackers.
The addresses were contained in a database of subscribers to American consultancy Stratfor which also featured encrypted versions of the passwords used to access its website.
Cyber-security expert John Bumgarner has identified the email addresses of 221 Ministry of Defence officials, 242 Nato staff, 67 Scotland Yard and other police personnel, 45 civil servants from the Foreign Office, 14 from the Home Office, and seven from the Cabinet Office.
British Government sources stressed that the breach was "not a big deal".
Labour MP Jeremy Corbyn was among 23 people who work in the Houses of Parliament whose details were obtained.
The database contained thousands more email addresses, of mainly Americans, including 19,000 US military.
It was exposed by hackers, believed to be part of the Anonymous group, who gained access to the records over Christmas.
Stratfor, which is based in Texas, specialises in foreign affairs and security issues. It had the details of user IDs - mostly email addresses - and encrypted passwords stored in spreadsheets.
Mr Bumgarner, of Washington research body the US Cyber Consequences Unit, told The Guardian: "Any foreign intelligence service targeting Britain could find these emails useful in identifying individuals connected to sensitive government activities."
A British Government spokesman said: "We are aware that subscriber details for the Stratfor website have been published in the public domain.
"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."