Always Wanted to be Secret Agent? Here’s How You Can Join Britain’s MI5 MI5, also known as the Security Service, is Britain’s domestic counterintelligence and security agency, with its chief mission being focused on the protection of the United Kingdom and its citizens. MI5, also known as the Security Service, is Britain’s domestic counterintelligence and security agency, with its chief mission being focused on the protection of the United Kingdom and its citizens. If this job description is intriguing, you should know that you must be at least eighteen years old and a British citizen—and one of your parents must be a British citizen or hold one of the following nationalities or citizenships: British Overseas Territories, British subject, British National (Overseas), British protected person, citizenship of the Commonwealth, citizenship of an EEA nationality, or citizenship of the United States of America.