)[22] The founding head of the Army section was Vernon Kell of the South Staffordshire Regiment, who remained in that role until the early part of the Second World War.

According to the official history of MI5, the actual number of agents identified was 22, and Kell had started sending out letters to local police forces on 29 July, giving them advance warning of arrests to be made as soon as war was declared.

Portsmouth Constabulary jumped the gun and arrested one on 3 August, and not all of the 22 were in custody by the time that McKenna made his speech, but the official history regards the incident as a devastating blow to Imperial Germany, which deprived them of their entire spy ring, and specifically upset the Kaiser.

[34] Using DMP detectives Ned Broy and David Nelligan, Michael Collins was able to learn the names and lodgings of the M04(x) agents, referred to by IRA operatives as "The Cairo Gang".

[32] That afternoon, a mixed force of the British Army, the Royal Irish Constabulary, and the Black and Tans retaliated by indiscriminately shooting dead 14 civilians at a Gaelic Football match at Croke Park.

[35] The net impact of Collins's strike of Bloody Sunday, 21 November 1920, was relatively negligible, even though the IRA had not gone up against MI5 professionals, but instead only against a quickly trained outfit of amateur army "D-Listers".

Within days, the remaining 160-odd M04(x) agents were re-established in secure quarters inside solidly loyalist hotels in Dublin, from where they continued to pursue Collins and the IRA relentlessly right up until the truce of July 1921.

Shortly thereafter, in a private meeting with Prime Minister David Lloyd George, Sir Basil Thomson was sacked, and the Home Intelligence Directorate was formally abolished.

It continued to think in terms of agents who would attempt to gather information simply through observation or bribery, or to agitate within labour organisations and the armed services, while posing as ordinary citizens.

The most successful of these agents; Harold 'Kim' Philby, Donald Maclean, Guy Burgess, Anthony Blunt, and John Cairncross; went undetected until after the Second World War, and became known as the Cambridge Five.

With the ending of the Battle of Britain, and the abandonment of invasion plans (correctly reported by both SIS and the Bletchley Park Ultra project), the spy scare eased, and the internment policy was gradually reversed.

[38] While the double-cross work dealt with enemy agents sent into Britain, a smaller-scale operation run by Victor Rothschild targeted British citizens who wanted to help Germany.

The 'Fifth Column' operation saw an MI5 officer, Eric Roberts, masquerade as the Gestapo's man in London, encouraging Nazi sympathisers to pass him information about people who would be willing to help Germany in the event of invasion.

[40] It is believed that two MI5 officers participated in "a gentle interrogation" given to the senior Nazi Heinrich Himmler after his arrest at a military checkpoint in the northern German village of Bremervörde in May 1945.

[43] The post-war period was a difficult time for the service, with a significant change in the threat as the Cold War began, being challenged by an extremely active KGB, and increasing incidence of the Northern Ireland conflict, and international terrorism.

A file was kept on Labour Prime Minister Harold Wilson from 1945, when he became a Member of Parliament (MP), although the agency's official historian, Christopher Andrew maintains that his fears of MI5 conspiracies and bugging were unfounded.

[49] One of the most significant and far-reaching failures was an inability to conclusively detect and apprehend the 'Cambridge Five' spy ring, which had formed in the inter-war years, and achieved great success in penetrating the government, and the intelligence agencies themselves.

[37] Related to this failure were suggestions of a high-level penetration within the service, Peter Wright (especially in his controversial book Spycatcher) and others believing that evidence implicated the former Director General, Roger Hollis, or his deputy Graham Mitchell.

Both men claimed to journalist Liam Clarke in the Belfast Telegraph that they were abandoned by MI5 and were "left high and dry despite severe health problems as a result of their work and lavish promises of life-time care from their former Intelligence bosses".

[53] The end of the Cold War resulted in a change in emphasis for the operations of the service, assuming responsibility for the investigation of all Irish republican activity within Britain,[54] and increasing the effort countering other forms of terrorism, particularly in more recent years the more widespread threat of Islamic extremism.

[56][57] In 2012, a document based review by Sir Desmond de Silva QC into the 1989 murder of Belfast solicitor Patrick Finucane found that MI5 had colluded with the Ulster Defence Association (UDA).

[58] Prime Minister David Cameron accepted the findings, and apologised on behalf of the British government, and acknowledged significant levels of collusion with Loyalists in its state agencies.

[60][61] During April 2010, the Real IRA detonated a 120 lb car bomb outside Palace Barracks in County Down, which is the headquarters of MI5 in Northern Ireland and also home to the 2nd Battalion The Mercian Regiment.

[63] Executive liaison groups enable MI5 to safely share secret, sensitive, and often raw intelligence with the police, on which decisions can be made about how best to gather evidence and prosecute suspects in the courts.

[65] Tasking was reactive, acting at the request of law enforcement bodies such as the National Criminal Intelligence Service (NCIS), for whom MI5 officers performed electronic surveillance and eavesdropping duties during Operation Trinity.

[71][72] In July 2006, parliamentarian Norman Baker accused the British Government of "hoarding information about people who pose no danger to this country", after it emerged that MI5 holds secret files on 272,000 individuals, equivalent to one in 160 adults.

It had previously been revealed that a 'traffic light' system operates:[73][74] In March 2018, the government acknowledged that MI5 officers are allowed to authorise agents to commit criminal activity in the UK.

Maya Foa, the director of Reprieve, said: "After a seven-month legal battle, the prime minister has finally been forced to publish her secret order, but we are a long way from having transparency.

[16] In November 2019, four human rights organisations claimed that the UK government has a policy dating from the 1990s to allow MI5 officers to authorise agents or informers to participate in crime, and to immunise them against prosecution for criminal actions.

[79] After the First World War, it relocated to smaller premises at 73–75 Queen's Gate in 1919,[80] and then moved to 35 Cromwell Road in 1929, before transferring to the top floor of the South Block of Thames House on Millbank in 1934.