MI5 launches instant terror alerts by email

The warning messages will be emailed whenever the official terrorist threat level rises to "critical" - meaning that details of a specific plot have been discovered and an attack is expected.

The aim of the move is to provide people with swifter information about the terrorist threat and to give those who might be affected more time to respond to the potential danger-and disruption. The public will be alerted as soon as the Government raises the alarm about an attack and within hours - or even minutes - of the Prime Minister and Home Secretary being told of the plot.

MI5's decision comes amid continued concern about the risk of another terror attack following the 7/7 bombings in which 52 innocent victims died. Dame Eliza Manningham Buller, the security service's director general, warned recently that 30 suspected plots and as many as 1,600 individuals are being monitored.

Tony Blair, John Reid and Met Commissioner Sir Ian Blair have also highlighted the potential risk and the official threat level remains "severe", which means that an attack is regarded as highly likely.

Although some observers might be concerned that the new emailed warnings will heighten fears, security sources say the system was a response to public demand for clear information on the terrorist threat. "We get a lot of emails from people wanting to know about the terrorist threat and this is one way in which we can engage with the public and provide up to date information," said one source.

"The threat levels are already published on the MI5 website but this is a way of making the details available more quickly. There is also plenty of security advice on the website telling people what they can do to protect themselves."

The publication of the threat level - which is assessed by the Government's Joint Terrorism Analysis Centre - began last August and the information is also displayed on the Home Office website.

Five classifications are used, starting at "low" and rising to " moderate", "substantial", "severe" and "critical". The last time the level was classed as "critical" was during last summer's Heathrow terror alert over an alleged plot to blow up transatlantic airliners between Britain and the US, for which 15 people are now facing charges.

Under the email alert system, which is due to begin this week, any member of the public who registers their email address on the MI5 website will receive notification whenever the threat level changes.

Although, an alert would also be sent if the threat drops from "severe", sources say it is " inconceivable" that this will happen because of the large number of suspected plotters in the UK.

People registering on the MI5 website will also be able to obtain instant email updates when speeches by senior politicians or the security service's director general or details of terrorist convictions, are posted on the site.