Britain's MI5 domestic spy agency has begun a new email alert service to warn the public about changes in the security threat level.
Internet users will be able to register on the MI5 website to receive automatic electronic updates in their email inboxes.
The email alerts are the latest in a series of moves by MI5 and its partner, the international spy agency MI6, to open up to the public after decades of guarding extreme secrecy.
In recent years both MI5 and MI6 have set up websites offering security advice and information about careers in the spy services, and have even run recruitment advertisements in newspapers.
The current security threat rating is "severe", the second-highest level - indicating the government believes an attack is highly likely.
The head of MI5, Eliza Manningham-Buller, said last November there were at least 30 active Islamist militant plots in Britain and that threats could involve chemical or nuclear devices.