The Security Service, which is responsible for our nation's counterterrorism efforts, plans to send out email alerts to citizens warning them of changes to the "national threat level", a measure of the risk of terrorist attacks. It will announce the start date and details of the signup process soon, it said today.

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 1 August 2006. It opened at 'severe', and has remained at that level ever since, save for the period 10 August to 14 August, when it rose to "critical" – after the police had arrested a number of people allegedly plotting to carry bombs onto aircraft flying to the US.

The Security Service, also known as MI5, gave little 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. 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 MI5 website said today.

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 not 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.

Unlike the UK, 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 this country, on the other hand, it's unclear how citizens are expected to respond to the email alerts. 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 on its website.

When the UK 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."