Behind the MI5 myth

Dame Stella Rimington became the first public face of MI5

The Security Service, better known as "MI5", is the UK's internal intelligence agency.

Its remit is to protect national security, which, in the late 20th Century encompasses a number of duties, though principally counter- terrorism.

Other responsibilities include espionage and industrial espionage (such as computer security), counter proliferation of weapons of mass destruction and, since 1996, assisting police at the higher end of their duties.

Set up 90 years ago, to investigate the threat posed by Germany, the service has undergone some radical changes in the post Cold War climate.

Much has been in the name of transparency and greater accountability. Until fairly recently MI5, or "Five" as it is known in the trade, operated as something of an open-secret - its existence widely known but not formally recognised.

Headquarters: Until recently a secret, the address is now widely published

Lord Denning's 1963 report into the Profumo affair revealed publicly, for the first time, details of the organisation's role and responsibilities.

The 1990s have brought some landmark reforms, including a published address, a public hotline for informants and a Website.

But the call by MPs for greater public scrutiny implies that much still remains hidden. The result makes it difficult to sift rumour from reality.

Early history

In 1997 the release by the Public Records Office of the service's earliest historical archives revealed much about the origins of MI5.

It was established as the Secret Service Bureau in 1909, to root out German spies operating in the UK. In 1916 the service became part of the new Directorate of Military Intelligence, hence the name change to MI5.

In 1931, it changed to the Security Service, but "MI5" has stuck.

The Cold War, with its web of spying and double agents, helped foster a racy, James Bond-style image for the organisation.

But by the late 1960s, East-West relations had begun to thaw, and the service redirected itself at the emerging threat of terrorism.

A notable turning point in the organisation's secretive history came with the high profile appointment of Stella Rimington - the first director general of the service to be publicly named.

It was a sign of things to come. Mrs Rimington ushered in an era of reform, holding press conferences and working to publish a recruitment brochure for graduates.

Previously, recruitment had mostly been down to a discreet "tap on the shoulder" and a chat over a glass of sherry.

The reforms have continued under Dame Stella's successor, Stephen Lander. But critics suggest they are little more than window dressing and questions persist about MI5's accountability.

The newspaper revelations of David Shayler, who quit the service in 1997, have damaged the service and the government, although they have never been proved.

Mr Shayler portrayed MI5 as a slow-witted, blundering organisation which thrived on its lack of accountability. He alleged incompetence had failed to prevent the bombing of the Israeli embassy in London in 1994 as well as IRA attacks in mainland Britain.

He also claimed files had been held on several high-ranking politicians including the Home Secretary, Jack Straw.