MI5 plans to send out email alerts to the public warning them of changes to the "national threat level", a measure of the risk of terrorist attacks.

As the UK security service responsible for the country's counterterrorism efforts, it said yesterday it will announce the start date and details of the sign-up process soon, but gave little detail away about its plans.

"Email alerts of changes to the national Threat Level and updates on the Security Service website will be available in the near future," the MI5 website said Tuesday. “This will enable subscribers to keep informed of major developments in national security affairs. You will be able to subscribe via a form on the Security Service website. We will publish an update shortly giving the address of the subscription form.”

The government already publishes its assessment of the current terrorist threat level online, rating it as one of five levels: low (an attack is unlikely), moderate, substantial, severe (an attack is highly likely) or critical (an attack is expected imminently).

It first made the threat level assessment public on last August, opening at "severe". It has remained at that level ever since, save for the period just after the first assessment publications when it rose to "critical" after police arrested a number of people allegedly plotting to carry bombs onto aircraft flying to the US.

The US Department of Homeland Security introduced a similar system in January 2003, using a colour code to rate the risk of a terrorist attack: low (green); guarded (blue); elevated (yellow); high (orange); and severe (red). The department has never dropped its threat assessment below "elevated" since the system was introduced. Following the arrests in London in August, it too raised its assessment to the maximum.

But unlike current UK practice, the US publishes guidelines on what changes in the threat assessment may mean for the general public. For example, a high level of risk in the US may result in cancellation of public events or the closure of government facilities all but essential personnel. A severe level of risk may lead to the closure of public buildings and public transportation.

In the UK, it's unclear how citizens are expected to respond to the email alerts, as the security measures that may be taken in response to the different threat level assessments "will not be announced publicly, to avoid informing terrorists about what we know and what we are doing about it," MI5 said.

When the government began publishing its threat level assessments in August, security expert Bruce Schneier repeated his criticism of the US system in his blog: "A terrorist alert that instills a vague feeling of dread or panic, without giving people anything to do in response, is ineffective. Indeed, it inspires terror itself."