The MI5 consists of 3,000 staff with its headquarters at Themes House in central London. They also have eight regional offices around the UK plus a Northern Ireland headquarters. The Security Service (MI5) is responsible for "protecting the UK against threats to national security from espionage, terrorism and sabotage, from the activities of agents of foreign powers, and from actions intended to overthrow or undermine parliamentary democracy by political, industrial or violent means"(Security Service website, 2008).

The MI5`s work focuses on investigating suspect individuals and organisations to assess secret intelligence relating to threats. They act to counter the source of threats, compiling evidence that enables them to bring their suspects to justice. They also advise the British Government and others informing them of threats and advise them on the appropriate response, including protective security measures. The MI5 also assist other agencies, organisations and Government departments in combating threats. They contribute to the collective work of the UK's National Intelligence Machinery and form partnerships with other organisations in the UK and abroad.

Timeline of major Terrorism in the 1990`s and 2000`s

•    1990 - The Provisional IRA (PIRA) assassinates Conservative MP Ian Gow at his home in Sussex.
•    1991 - PIRA attacks Downing Street with mortar bombs during a meeting of the Cabinet.
•    1993 - A huge PIRA bomb explodes in Bishopsgate, London, killing two people and causing damage costing over £350 million.
•    1994 - PIRA declares a "cessation of military operations" and Sinn Féin enters the ongoing Northern Ireland peace talks.
•    1996 - After a breakdown in talks, PIRA resumes its attacks in Northern Ireland and the mainland, bombing London's Docklands and Manchester city centre.
•    1997 - PIRA declares a second ceasefire.
•    1998 - Al Qaida vehicle bombs explode outside United States embassies in Kenya and Tanzania, killing over 220 people and injuring over 4,000 more. In Northern Ireland, 29 people are killed and over 200 injured when the dissident Real IRA attacks Omagh.
•    2000 - The Greek terrorist group November 17 assassinates Brigadier Stephen Saunders, the British military attaché to Greece. In the subsequent investigation, the group's leaders are captured and convicted.
•    2001 - Al Qaida operatives attack the World Trade Center and Pentagon using hijacked aircraft, killing themselves and nearly 3,000 other people. In response, the United States and an international coalition invades Afghanistan to expel al Qaida and its Taliban supporters.
•    2002 - 202 people are killed in an al Qaida-linked attack in Bali, Indonesia.
•    2004 - Terrorists attack Madrid commuter trains, killing 191 people and injuring more than 1,500.
•    2005 - Suicide bombers attack the London transport network, killing 56 people and injuring over 700.

The Mi5 was founded in 1909 and was then known as the Home Section of the Secret Service Bureau. It was established to combat Imperial Germany's espionage operations in the United Kingdom. During the four years of World War I, the Secret Service Bureau was in the front lines of the fight against German espionage. It was mobilised as a branch of the War Office under the Directorate of Military Operations and came under the branch known as MO5. The MO5 was renamed to MI5 in January 1916 and was incorporated into a new Directorate of Military Intelligence, and the Foreign Section became the Secret Intelligence Service (sometimes referred to as MI6).

Between the two World Wars, the MI5 faced the increasing challenge of espionage from the Soviet Union and Nazi Germany, as well as the internal threat of subversion by communists and fascists. The MI5 was renamed the "Defence Security Service" in 1929. Two years later, in 1931, it became the Security Service, the title that it still uses today. In 1931, the Service gained formal responsibility for assessing all threats to the security of the UK, apart from those posed by Irish terrorists and anarchists, which remained the responsibility of the police.

During World War II, the Security Service played a key role in combating enemy espionage, intercepting German communications and feeding misinformation back to Germany. The Service achieved great success in uncovering enemy agents in Britain, some of whom were "turned" by the Service and became double agents who fed false information to the Germans concerning military strategy throughout the war. This was the famous "Double Cross" system, a highly effective deception that contributed to the success of the Allied Forces landing in Normandy on D-Day in June 1944. When captured German intelligence records were studied after 1945, it was found that almost all of the further 115 or so agents targeted against Britain during the course of the war had been successfully identified and caught. The only exception was one agent who committed suicide before capture.

For the first time, the Service also confronted the issue of loyalty of British Communists in sensitive government positions, in the certain knowledge that Party members were under instructions to "share" their information with Communist Party HQ. This was to have important repercussions following the war.
In the aftermath of the defeat of Nazi Germany, concern grew in Western Europe and America about the extent of the Soviet Union's influence and ambitions. The war had ended with Europe divided between the two superpowers, the United States and the Soviet Union. In March 1946, the former British Prime Minister Winston Churchill memorably described how an "iron curtain [had] descended across the continent".

The history of post-war Europe, and indeed that of the Security Service, was dominated for the next thirty years by the ideological conflict between the Soviet bloc and the Western democracies. Subversion and Soviet espionage were key concerns during the Cold War, and from the 1970s onwards terrorism also emerged as a serious threat to national security.
Major changes in the focus of the Service's work took place in the early 1990s with the end of the Cold War. The threat from subversion diminished greatly with the demise of the Communist bloc, to the point where it is now considered negligible. The threat from espionage persisted but by this time required fewer resources to tackle.

Terrorism, however, had not abated. Large-scale Provisional IRA attacks continued to take place in Northern Ireland and the British mainland, and international terrorism posed a growing threat worldwide. In October 1992 responsibility for leading the intelligence effort against Irish republican terrorism on the UK mainland was transferred to the Service from the Metropolitan Police. The Police Service of Northern Ireland (formerly the Royal Ulster Constabulary) retained this responsibility in Northern Ireland.
In this new work, they drew on the experience gained from their role in running long-term intelligence operations against Irish-related terrorism overseas during the 1970s and 1980s. The Service came increasingly into the public eye during the mid-1990s, with MI5 officers giving evidence in a number of Irish Republican terrorist trials.

The 1994 Intelligence Services Act further strengthened the Service's legal basis. It also established the Intelligence and Security Committee, a committee of senior parliamentarians charged with overseeing the expenditure, administration and policy of the three intelligence and security agencies (the Security Service, the Secret Intelligence Service and the Government Communications Headquarters - GCHQ). The Security Service Act was amended in 1996 to give the Service a role in assisting the law enforcement agencies in fighting serious crime.


Source:  MI5 Security Services website © Crown Copyright 2007