LONDON - Britain's MI5 will begin sending out terror alerts to members of the public over email on Tuesday, part of a move by the spy agency toward greater openness.

Anyone interested in following the threat level -- which has sat at "severe" since Aug. 14, 2006 -- will receive email updates should the level change. The highest level is "critical," meaning an attack is imminent.

A second service allows users to subscribe to updates on MI5's website, which might include news about leadership changes or the government's agenda for combating terrorism, Home Office spokesman Stephen Carter said.

The email alert system is another tentative step into the limelight for the domestic spy agency, whose doings -- and even existence -- has long been shrouded in secrecy. The government only officially acknowledged the existence of MI5 in 1989, and its leadership remained a secret until 1992.

It unveiled its terror alert system last August. The threat level has risen once since then, rising to "critical" on Aug. 10 after the announcement of a foiled airline bombing plot. The threat level fell back to "severe" five days later.

The United States and France, which have similar alert systems, do not offer any kind of official email service.