The secret service agency is putting in elaborate security measures to stop computer hackers breaking into its new web site and using it to spread false information.
MI5's move into the Internet, which is due to take place by the autumn, is part of the service's attempt to promote its policy of greater openness.
Although, at first, the site is not expected to be open to inquiries from the public, a postal address will be published. Later, the agency will consider opening an e-mail address for people around the world to make contact. This could be a useful source of intelligence and anonymous tip-offs.
There will also be details about recruitment into the service. Other information expected to be electronically published includes details of MI5's work, such as targeting the IRA and Russian spies, its budget, manpower, aims and history.
In the United States, the Federal Bureau of Investigation has used the Internet to help in its inquiries. Last year, it received thousands of responses after it publicising its e-mail address and asking for help in solving the mysterious crash of TWA flight 800 in which 230 people died.
MI5 is highly unlikely to make such direct appeals, but important details could be sent anonymously about things such IRA activities, by informers or citizens who are too frightened to be identified. The police and security services have had a series of successes during the past year on the strength of tip-offs from the public.
But before the web site is launched security chiefs want to ensure the system is tamperproof. Computer experts are currently working on a noticeboard that cannot be hacked into and altered.
The ease with which some hackers have been able to break into sites was demonstrated to devastating and embarrassing effect in December when the Labour Party's world-wide web site was attacked. An American computer "geek" broke into the site and changed the title "Road to the Manifesto" to "Road to Nowhere". He also tinkered with links to other web sites so they read: "The Labour Party sex shop," and transferred visitors to pages carrying pornography. Later Mr Blair's Spitting Image effigy was added under the banner "Hacked Labour: Same Politicians, Same Lies."
On a more serious note, hackers on the Internet broke into US Defense Department computers more than 160,000 times in 1995 investigators from the Congress discovered.
Sinn Fein supporters at the University of Texas have also been accused of publishing a terrorist "crib sheet" on the Internet giving detailed information about alleged MI5 installations and military bases in Northern Ireland.
The very nature of the Internet - an open system without a governing body - encourages hacking. However, a number of software security packages called "firewalls" and other techniques for keeping out unwanted visitors have been developed.
During the past few years, MI5 has raised its public profile by advertising for potential recruits and publishing an annual report, which gives a wide range of details about the service. Other security agencies such as MI6 have yet to decide whether to follow suit.